Anyone saying that the Tamron 90mm f2.8 is nt as good as the Nikon 105mm or Canon 100mm does not know what they are talking about. The USM of the Canon is very nice and fast though, but does not improve image quality. The Tamron tops the Nikon, and also it is a lot nicer to use (easy switching between manual and auto focus, very nice manual focus mechanics (very precise to operate).
If you need to be careful with your budget, get a 50mm Sigma. If you want a the best lens for under 500$, get the Tamron 90mm. If you want a Nikon, get the 60mm f2.8 micro. It bests the 105mm in image quality.
Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30