Did you ever figure out your question of whether it is better to get a Mac-formatted drive? I have the same question.
In regards to your original question, I have a Seagate FreeAgent that works between my mac and my PC - I partitioned it into two drives. One is FAT32 for the PC and the other one is for the MacBookPro. I can get the files from the FAT32 to the MBP, but not the other way, and I'm primarily dealing with pictures. But it uses a firewire and it's own power, so it's not very portable. I'm looking at the Passport Elite 500G (on sale at Costo for $99 with a coupon right now) but there is another one that is "Mac Formatted". I assume that these were the ones you were looking at also! My plan is to leave the Seagate as is, and use the portable as a redundant drive only. Anyway, would love to hear what you found out, or what you did.