I trust the New Zealander who reviews at CameraLabs.com. He says this, comparing the two:
[quote]
Compared to Canon Rebel XSi
Physically speaking the new EOS T1i shares a great deal with its predecessor, the best-selling EOS XSi. They share the same dimensions, virtually the same weight (the new model’s 5g heavier), the same viewfinder and AF specification, and even the same battery (albeit now with 20% shorter life). The continuous shooting speed is also virtually the same, although the new model makes up for fractionally slower speed with a larger frame burst.
In its favour, the new EOS 500D / T1i boasts an extra three Megapixels, a much more detailed VGA monitor, an HDMI port, a much higher maximum sensitivity of 12800 ISO compared to just 1600 on its predecessor, Peripheral Illumination Correction to reduce the corner darkening of lens vignetting, the Creative Auto mode, and of course the headline feature of HD video recording.
In our tests, we didn’t notice much difference in terms of real-life detail, especially when the 500D / T1i was fitted with the standard EF-S 18-55mm IS kit lens, although there’s the potential for superior resolving power if you fit better optics. Noise levels were similar at lower sensitivities, although at 800 and especially 1600 ISO, the 500D / T1i avoids the chroma artefacts of its predecessor. But in terms of overall image quality, there’s not much in it.
So you should really only choose the 500D / T1i over the 450D / XSi if you’re either coupling it with decent lenses, or value its new features, like the HD Movies and VGA screen. If you’re not bothered by the movies or screen and intend to use the kit lens, then save yourself some money and go for the older 450D / XSi. It remains an excellent DSLR which is likely to fall in price further as stocks last. See our
Canon EOS 450D / XSi review for more details.[/quote]
Source:
http://www.cameralabs.com/reviews/Canon_EOS_500D_Digital_Rebel_T1i/index.shtml