What are the advantages and disadvantages of negative/print film compared to reversal/slide film?
I started with (color) negative film then switched to slide film for one specific reason: I wanted control over my pictures. With negatives, unless you develop and print yourself (which is a convoluted process for color film), when you drop your film at the local shop, you have almost no control over the look of your prints (exposure, contrast). And with negative film, it's hard to judge how good your picture is without a print (negative has crazy colors).
With slides, it's very simple: the process is the same for all slide film, the result is always the same, and the slide you see is 100% your picture, your work. If you under- or over-expose (on purpose or by mistake), it's your work, not that of the machine that made your prints. Plus, colors on slides, especially saturated films (eg. Velvia), looks worlds better (to me), but that's a matter of taste.
Also, when developing slide film, prints are extra, but I don't spend money for any print up front: I select those pictures I like and scan them, they can be printed like any digital picture.
On the downside, slide film is much less forgiving of bad exposure: when light is very contrasted, you have to choose if you sacrifice highlights or shadows (similar to digital), while negative film can take more contrast. It's also getting hard to find slide film in brick-and-mortar shops (I buy all of mine online), so when travelling you better bring enough film. Also, scanning film to have prints is more work, and good scanners for slide or film are more expensive than entry level scanners.
I prepared this as an answer to a question which marked as a duplicate of this one, and not wanting to lose it, I'll post the answer here:
Firstly, the differences between them.
Negative film is sometimes called print film. It's useful when you're wanting to make prints of the image as when you shine light through it in an enlarger/projector onto a material that gets darker when more light is applied (like a negative or print paper) then you get a positive image as a result. Negative films typically have a higher exposure latitude meaning they're tolerant if you don't get the exposure just right and can still record detail. Slide film is much more sensitive to a correct exposure. This ability to use an enlarger to make a print provides much greater after-the-fact control over contrast and localised brightness in the resulting print. Negatives often appear when held under light to have a much lower contrast compared to slide film, but when viewing the negative as a stepping-stone on the path to a print where you have contrast control, this is not as bad as it seems. Modern colour negative film is typically developed in the C41 process.
Slide film is more typically used in projection onto a surface for viewing. Reproducing it chemically is quite difficult - there used to be ways of doing it using no-longer-available Ilfochrome or making an internegative - and as such prints are usually made digitally. When digital scanning was not yet wide spread slide film was usually a last resort if you wanted to make a print. That said, slide films are often popular as they have great brilliance and colour saturation compared to negatives as well as typically a very fine film grain. They are higher contrast (and hence have a smaller exposure latitude) which is great on dull days but can lead to blown shadows or highlights on bright sunny days. For this reason many people underexpose slide film by a third or a half of a stop. This added difficulty of getting a good exposure should not be considered bad, the more unforgiving nature is a good way to start learning how to measure light correctly. Modern slide film uses the E6 process which has more steps in developing than C41 and as such takes longer as there are more baths to use. It's sometimes more difficult to find a lab that can process E6, although kits are available from companies such as Tetenal if you want to try it at home.
From the 35mm handbook:
Slide films are typically colour balanced for daylight illumination or for tungsten light sources used in projection and used to be available in a wide range of speeds due to the contrast requirements. The sensitivity of slide film to contrast means that scanners need to configure their lamp brightness more accurately than with negative film, but this is not a problem in modern life as it is handled for us in the scanning software/hardware. Indeed, it is these twin properties of higher contrast and greater reproduction of vivid colours that mean scans of slide film typically seem to need less post-production work or are ready out-of-the-box.
The 35mm handbook also mentions that slide film suffers less from colour distortion with exposures longer than one second.
That said, the technical discussion somewhat breaks down some of the fun of experimenting with film. For example, you can get incredibly saturated colours by shooting colour negative film and developing it in E6, the slide processing chemicals. This process is called Cross Processing or XPro and is a fun technique to try out.
Note: I also checked the Ilford Manual of Photography Sixth Edition, Focal Press, 1971, but it barely mentions transparency film with the exception of their use in Magic Lantern Projection shows. While negative-positive processes have been around since Henry Fox Talbot, at the same time (1839) in France Louis Daguerre announced his daguerreotype positive process which initially was much more widely spread. Positive-negative processes gradually became more popular after around 1860, the daguerreotype had pretty much died out in popularity due to the long exposure times were impractical in a time when there was a flourishing market for portrait photography.
Another difference - negative film is darker than slide film and is therefore harder to scan properly. You need a quality scanner with a strong light source to properly scan negative film. (Of course, you need a quality scanner to properly scan dark parts of a slide film but sometimes you can handle loss of detail in dark areas.)
Reversal film has a smaller exposure latitude than negative film, so it's harder to expose correctly. Also, as there is an extra step from negative to print, a negative that is slightly under- or overexposed can be corrected.
The extra step of printing negative images can also be used to control the contrast of the photo, by using papers with different contrast grade. This is mostly used in black and white photography, though.
There is of course also the difference in the final result. Reversal film can also be printed on paper, but it's mostly used as slides. Photo prints can easily be viewed without any equipment, while you need a viewer or projector to view slides. On the other hand, slides can be projected very large, while large prints are very expensive.
One of the key disadvantages of slide & transparency film is the ability to casually browse the results.
My father preferred slide film for many years, for its quality, and as a result, many of his family photos are locked away in boxes of slides, unseen for years, while his prints are available for browsing in an album at any time.