I'm finally getting a DSLR! I'm deciding between two Canon Rebels - the XS and the XSi. How useful are the XSi's extra features? I know I don't need more than 10 megapixels, but I don't know enough about the other extra features to know how necessary they are. I can afford either, so which would you recommend getting? Save some money and get the XS, or just buy an XSi?