Depends on you and what you want to do with it.
If you want to shoot videos you need the Canon Digital Rebel T1i
Otherwise the Canon EOS 50D
has more features such as microfocus adjustment, a 1/8000s shutter speed, a better AF system with 9 cross sensors, two LCDs, a rear QCD (Quick Control Dial), a faster continuous shooting rate with a larger image buffer and a larger viewfinder with a true pentaprism. If you get the 50D, the deal with the EOS 50D body plus 28-135IS lens
is pretty good since the lens then only costs you around $200.
The 28-135 obviously isn't as wide as the 18-55, so that may factor into your lens choice too.
The T1i is maller and lighter, but the 50D is more sturdily built and for me the slightly larger 50D is easier to hold.
So it really comes down to what you want (especially the video feature) and what you can afford.