I own the the T2i (purchased it for video), but have also done professional photography with it. 18 megapixels are more than enough for poster prints, but the sheer number of pixels don’t say anything about the picture quality. Don’t fall into the trap of comparing cameras based on pixelcounting.
I’d recommend to go to the Canon website and do a comparison of the various models. You can then see more clearly what the different models provide and decide what you really need.
The T3i is a great stills camera but has fewer features than the more professional models. But from a picture quality view IMHO all the APS-C models create comparable pictures if you use the same lenses.
Bear in mind, that if you purchase a smaller model, you’d maybe have more money to spend for lenses. And lenses are the single most important quality factor of a camera. You can change the camera body later but keep the lenses.
For *my* purposes (and the very few professional stills shoots I had/have) the smaller camera is great.
Just my two cents
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