Hello and welcome to the Cameralabs forums.
Yes, you're right, the Sony has a rebranded Minolta mount, so should take any Minolta lens, but there are a larger variety of Canon lenses available.
That said, few people would own more than a few lenses, so the number which are available is rarely an issue - whats more important is if the particular lenses you desire are available, and also if they are within your budget.
It's certainly a tough choice though deciding between the Canon Rebel XT plus, say, EF-S 17-85 IS, versus the Sony A100 with 18-70mm. Both are excellent packages.
We'd advise going over our reviews and results for all these products, but crucially going into a store and picking both up to see which feels better in your hand. Since both products deliver excellent results, their actual look and feel should decide it for you.
Also bear in mind Canon will almost certainly announce the Rebel XT's successor in the next month, so we'd recommend waiting to see what comes out before making a final decision. If you're looking at the Sony A100, you should also consider the Nikon D80.