I think that I'll finally be entering the eReader/tablet world next year now that the Kindle Fire and Nook Tablet will be available. DC going to the Kindle and Marvel to the Nook is annoying, though. My decision on which device to choose will be based in part on which comics I'd rather go with.
I prefer Marvel overall, but I'm more interested in their past issues than I am in current ones. I think read more Vertigo than either mainstream DC or Marvel, so the Kindle would probably be better for me, but it would also depend on the availability of Dark Horse and Image comics.
I don't really know how the digital comics system works; does Comixology offer comics from Dark Horse and Image? Will Comixology be supported on both devices?