Hey guys, I'm looking for a flash for my Rebel XT. I was thinking either a 430EX or something made by Sigma or another third party that is around the same price. I don't plan to use the flash for anything with slaves or whatnot--just a standalone flash for shooting people in dark areas or as a fill flash. What are the advantages and disadvantages of using a Canon or third party flash? Also, what does something like "90' @ ISO100" mean? Thanks.