consider renting both for a week or 2, try them out and see which you prefer, the 50 does get a little close, equals 80mm field of view on crop camera, but does focus pretty fast, i rode great movie ride with tim and used his 50, focus nice and fast, but i liked the sigma a lot
i rented the sigma and a canon 70-300 IS for 2 weeks for mousefest, from this outfit,
http://www.lensrentals.com/ and was very pleased, i think it was $134 for both lenses, shipping and insurance, they came well packed and padded and with a peel off and stick shipping label
we are going on the magic, 1st med cruise on 5/26/07 and i'm considering an xti body and the sigm,a for museum/church pics, that plus my 10-22 and my velbon tripod are all i'll take, i may rent the 70-300 again for the trip, when i get my mec sling pod next month i'll be able to judge how much gear i can fit into it for daily travels
all my posted potc, splash mtn, finding nemo, and much of gmr pics were with sigma at 1600 iso, i took a few at 3200, but mostly fantasmic with 70-300 which is f3.5 at widest
hope this helps
gary