I currently own a 600d + a 18-135 kit lens.. but one thing i fell missing is reach.. I can't afford an expensive lens like the 2.8 is or the 100-400 so i thought about two lenses: canon ef 70-200L f4 canon ef 70-300 f4-5.6 is I just want to know if there is any noticeable improvment from my kit lens to these two and the differences between those two because if the overall performance are close i prefer a longer lens. thx.
You asked about a few things:
Firstly the IQ improvement is huge between L and non-L. However what does image quality mean to you? To some, it means getting paid or NOT getting paid. For hobbyist, it usually does one thing :
200mm is not a much longer than 135mm. If you can, go try it yourself, you will STILL think 200mm is not long enough. As you take more photos, you start to realize you can create good photos at all focal lengths, unless when shooting specific subjects (like tiny birds).
Again, for people who needs to protect their reputation as professional photography whose living depends on image quality (and many other things), getting an L lens is a no brainer, you don't want to go out of business just to save $2,000.
For own-use, however, the image quality is not so important. We are talking about THOUSANDS of dollars for better PIXELS, not better PHOTOS.
For a balance between the three, I recommend the 55-250mm lens, it is cheap, light, has 250mm(400mm equiv.)
You can have a photo that is sharp and crap, and have another one that is taken with a kit lens that touches your own or other people's hearts.
If you are talking about the 70-300 NON "L", then no, no improvement. If you are talking about the L version, then yes a HUGE improvement. The 70-200L will also be a slight improvement.
You don't mention which 18-135 you have(two current models), but any "L" lens will certainly be an improvement over either. I would take a look at the lenses in question on this website and I believe it will answer many of these questions for you:
You have to consider that all "L" lenses are built for full-frame cameras, not for APS-C cameras like the 600D. The 70-200mm have to be multiplied by the crop factor which is 1.6 (equals a range from about 110mm to 320mm).