Is the Canon Rebel XSi or the Canon Rebel XTi a better camera?

Canon Rebel XSi - for sure. It is newer than Canon Rebel XTi and has 12 MP while XTi has 10 MP.

Plus the XSi has live view, like a point and shoot camera you can use the LCD to take pictures.

=================== I personally like the Canon 50D more than xsi or xti