First up, is the Amazon Associate functionality. LibraryThing lets you use your own Associate ID, which means you get credit for any sales resulting from your links. As near as I can tell, Shelfari uses their own ID and there's apparently no way to override it. Bad idea.
Secondly, although Shelfari does indeed allow you to import your titles from LibraryThing (or any other service that exports to tab-delimited files), it ignores the review link field. That's one of the things I like best about LibraryThing: they let me point to my review of the book through their service. Shelfari apparently doesn't handle this through the import process and there's no way I'm going to manually enter all those links again! (Btw, LibraryThing includes the links in the export file, so they're available if Shelfari should choose to use them.)
So although I liked what I saw initially in this quick look at Shelfari, I'm sticking with LibraryThing for now.
UPDATE: The score now stands: LibraryThing: 2, Shelfari: 1. Mark Williamson of Shelfari has pointed out that I missed the Advanced tab options during the import phase. There's an option on that tab that lets you use your Amazon Associate ID, not Shelfari's. I still miss the review link feature though...