I shoot landscapes and weather/skies almost exclusively, and have been using an XSi for over four years now. For that type of work, I feel the XXD series and 7D offer relatively little to justify their price premiums, so I've mostly been content with what I have.
With T3i/T4i prices hitting new lows and the T5i announced, I'm thinking I probably ought to grab one before they're phased out. I'm just unsure whether to expect a noticeable improvement in IQ, particularly with regard to noise at higher ISO. Granted, there are several other new features of interest too, but not necessarily enticing enough to make me part with $500-600.
Does anyone know of a direct side-by-side comparison of the XSi with either the T3i or T4i under low-light conditions and high ISO? I shoot RAW all the time, so improvements in JPEG processing mean nothing to me. The jump from 12 to 18 MP also makes direct pixel-by-pixel comparisons a bit unfair, so it's really tough to quantify. Perhaps someone who's done this exact upgrade and shoots high ISO regularly can comment on their experience?
A majority of my exposures are at ISO 100 (tripoded if necessary), but I do sometimes dabble in night landscapes and star trails, where any significant noise improvement would be very welcome (example shot at ISO 1600). The 6D has me very tempted, I'll admit. Honestly, if I thought my 17-40L on FF would be the equal of my 10-22 in terms of corner sharpness and vignetting, I'd probably be convinced -- but reservations on that (and Canon's current lag behind competition in sensor tech) are likely going to keep me crop for now.