Considering T3i to 5D...
Well, the T3i has 50% more pixels, so that's an issue if you print big. The low-light performance of the 5d is definitely good, but not too much better than the t3i. You'll likely get 1 full-stop better noise in your high-iso shots.
You'll also lose a lot of features. For example, the 5d is a dust magnet, and you'll have to manually clean the sensor every so often. Also, there's no movie mode or live-view, the latter of which is very useful in the studio for composition and manual focusing.
Still, the 5d has some magical qualities about it. The pixel density is MUCH lower, so your shots will seem sharper with the same lenses while viewing at 100%. You'll get more control over depth of field, and your lenses won't be as affected by diffraction at the same apertures. Better build quality is also nice to have. And finally, many professionals claim that the 5d produces amazing colors and just has a "magic" feel in its images.
It's something you'll definitely want to try before you buy, as it's a step back in many directions, but huge leaps forward in others.
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