Canon EOS 600D body with other company's lens OR with kit lens

Question

What should be a wise decision to buy. I need a comparison of the two. This is going to be my only/first lens. You may provide any other suggestion.

[Edited]
a. A brief discussion about added (270 mm -135 mm) focal length at same price
b. A discussion about any possible difference that may exist between lenses of Tamron and Canon

Canon 600d body only with Tamron 18-270mm F/3.5-6.3 Di II VC PZD lens 

or

Canon 600D with kit lens 18-135 F/3.5-5.6

Q1 What would be the pros and cons given I will get extra mms with Tamron at almost same price.

Q2 Will the other company's lens fit to Canon 600?

[Edited]
while I am obliged with the answers received but my mention of the type of photographs I will take has ruined the whole sense of my question and hence I will write the below

I don't know but why everybody with some reps on the site always have to ask about the type of photographs a beginner would take, while they can easily escape that and assume pics don't need to be pro, he just want to learn.

I am removing wedding from the below list. Need: Basic start with DSLR, picnic shoot, friends gathering

Answer 1

It won't make a lot of difference for casual useage. The Canon EF-S 18-135mm f/3.5-5.6 IS will probably give you better image quality between 18mm and 135mm in terms of geometric distortion, vignetting, etc. They're both about the same in terms of sharpness at common focal lengths and apertures. Obviously the Tamron 18-270mm f/3.5-6.3 Di II VC will give you more reach at the expense of some image quality, but the sharpness of the Tamron above 135mm begins to degrade and by 270mm is fairly soft. Just how usable the extra reach on the Tamron will be is debatable.

Both these lenses will be fairly good when outside in daylight. Neither will be particularly good indoors without an off-camera or bounce flash. Either will allow you to learn the basics of how to operate your camera but neither will allow you much room to explore wider apertures and how they affect depth of field. For about the price difference between the 600D body + Tamron 18-270 and the 600D Body w/18-55 kit lens you will save enough to also buy an EF 50mm f/1.8 II and a generic flash such as the Yongnuo YN-560 II plus a generic off shoe cord or wireless radio trigger. That is what I would recommend. If the primary purpose of the purchase is to allow you to learn the basics of photography, the wide aperture prime will be worth far more to you than a narrow aperture, large focal length range zoom.

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As to the part of your original question regarding which of the two options would be better for shooting weddings:

None of the above. If you are serious about doing weddings you need better gear than either of the options you have included in your question.

You need a body capable of faster handling than the 600D with only one control wheel. The Canon semi-pro and pro bodies include a second wheel on the back of the camera and a higher number of dedicated buttons so that you can adjust many more parameters when shooting without taking your eye away from the viewfinder. While you can do a lot of the same things with a Rebel body, the time it takes to dig through the menu to adjust a setting will often mean the shot has been missed by the time the camera is ready.

You need lenses faster than either of your choices to shoot an indoor event such as most weddings. At a minimum you need a normal zoom with a constant f/2.8 aperture or a set of faster primes such as a 35mm f/1.4, a 50mm f/1.4, and an 80-85mm f/1.8. You also probably need a fast telephoto lens such as a 70-200mm f/2.8.

You also need to remember that you should never shoot a wedding with only one body available. Always have a back up body and enough lenses to cover the job should any one piece of equipment fail. You also need some off camera lights and modifiers for the formal shots.

Just from the perspective of camera body and lens, a typical wedding pro will have something like the following:

• Two bodies capable of fast handling and good high ISO performance. Something like a Canon 5DIII and the 5DII it replaced or maybe a 7D for the telephoto shots if there is enough light for the APS-C sensor.
• A fast normal zoom lens, such as a 24-70mm f/2.8, a fast 50mm prime, and a fast telephoto lens. For backup something like a 24-105mm f/4 will work in a pinch.

Answer 2

Any EF or EF-s mount lens will fit on the Canon 600D. Tamron, Sigma and several other manufacturers make lenses that are compatible with EF mount cameras and the 18-270 you are talking about should be compatible.

That said, while I don't know much about the kit lens, the Tamron lens is a super, super long super-zoom. Going from 18mm to 270mm means massive sacrifices in terms of image quality. As a general rule of thumb, the longer the range a lens covers, the lower the quality has to become, particularly when going all the way from wide angle to super-telephoto in a single lens. This can be overcome some by more expensive lens elements, but results in a significantly more expensive lens than lenses covering sub-sets of that range at the same quality. 6.3 is also an extremely slow aperture that is going to be completely unusable for anything fairly dimly lit, such as weddings. I'm not sure how far through it moves to f/6.3, but overall I'd try to avoid that lens.

For your interests, you best bet might actually be to go body only and then look in to something like a fast 50mm prime and a simply telephoto or super telephoto that is reasonably fast. Most of what you want to do seems to be candids and group portraits in mixed light and a decent, fast 50mm prime would serve you very well for that. Picnics and perhaps weddings would also make good use of a moderate telephoto lens, but to get the best quality, you'd want a dedicated telephoto, something like a 70-250 kind of range or maybe 55-225. You also don't really want any slower than f/5.6 on the long end or it won't be much use at a wedding, though it could still work well for picnics.