Well, IMO the 60D is expensive for what you get compared to the 50D, so unless you have need for video I'd pick the 50D.
http://snapsort.com/compare/Canon_EOS_50D-vs-Canon_EOS_Digital_Rebel_XSi
I would say the 50D justifies the higher cost over the Xsi in just about every way. It's a better built body with a larger viewfinder, better AF, better screen/live view, better FPS/IQ, more megapixels, larger battery. It's a different series of camera compared to the Xs and Rebel series, and likely worth the price difference over the Xsi.
I don't know where you're purchasing your Xsi from, but you can use the Canon Loyalty Program (get a broken point and shoot for 10$ off of ebay), and get the 50D for
http://shop.usa.canon.com/webapp/wcs/stores/servlet/product_10051_10051_211654_-1 562$ (CLP gets you 20% off refurbed cameras). If you don't think you need the better viewfinder/AF/build of the XXD series, the Rebel T2i can be had for around 450$ via CLP.
If it's less than a ~200$ difference between the Xsi and 50D, I'd almost definitely get it. The improved AF/viewfinder mean you're going to be getting more keepers, ignoring the image quality and feature benefits. If it's significant, it may make more sense to put that money into better lenses.