My first SLR: Canon XSi or something better?

I've always wanted to try my hand at learning photography for a hobby. Now that my 7-year old son is really showing an interest in it, I figure it's something we learn and do together. So, I'm looking at buying my first SLR camera. I'll probably take a class or two to learn some basics about photography.

Anyway, I'm looking at a few options. Money is tight, so the more I spend, the more of my electric guitar gear I have to sell. I hate to sell some of my gear too since I'm a worship pastor and use my electric gear from time to time.

I'm looking at a like new Canon XSi with an 18-55 EF-S lens and 55-250 EFS lens. Also comes with two batteries, charger, and case. The camera has just over 1,000 actuations. Cost is $425.

I found a great deal on a Nikon D90 kit with a couple lenses and a flash, but I don't really wanna spend $900. If I don't go with the XSi, I'd probably sell some gear and buy a new Nikon D5100.

The questions is as a beginner, should I stick with the less expensive XSi until I learn some basic photography skills and learn what I like and don't like in gear? Then, maybe next year at tax return time I could upgrade. The other option is to just buy the D5100 right now and roll with the kit 18-55 lens. Probably wouldn't be able to afford any other lenses for a while though.