Any Camera system will give you plenty of ability to produce for your potential clients, they all offer great quality. Which system matters much more to you than it does to your clientele. Cater the features and usability to your style, technique, and budget. The more important investment will be lens anyway. Stay away from the 70-300, you will find the 70-210 f4L will offer a lot more versatility and quality at a great price. I would even venture to say you'd be better off getting the XTi with the 70-210f4L than the XSi with the 70-300 f4-f6.3. It makes a lens incredibly more usable with a constant aperture, even at the sacrifice of a bit of range. 210 is not a weak telephoto though, by any means.

You may even want to check into Sony, as someone whos on a budget - the built in image stabilization saves you hundreds of dollars on lens.