First of all, your options for wide angle zooms are:
- Canon EF-S 10-22mm f/3.5-4.5 USM - 680$
- Sigma 10-20mm F4-5.6 EX DC HSM - 460$
- Sigma 12-24mm F4-5.6 EX DG ASPHERICAL/ HSM - 600$
- Tamron SP AF11-18MM F/4.5-5.6 Di II LD Aspherical (IF) - 530$
- Tokina AT-X 124 AF PRO DX (12-24mm) - 480$
you should take into account that canon ef-s, sigma dc, tamron Di and tokina dx are designed especially for APS-C size sensors (and for your EF-S mount on the 350D).
That means that you'll have considerably more distortion if you ever place one of these on a full frame body (if you'll ever switch to the 1D or 5D for example).
These lenses cost around 500US$ (give or take a 100$), and this is a lot of money for lenses you won't be able to use on your next cam.
On the other hand, most DSLRs made today have a smaller than full frame sensors, so even if you upgrade to a 30D or smthng like that, you won't have a problem.
All canon wide angle primes are for the EF mount, and not EF-S, all sigma's primes are DG and not DC, tamron's prime is not a Di and tokina have no wide angle prime at all.
By getting your self a prime, you avoid the dilemma above.
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