1. Have you played any other Mystery Dungeon games? If you liked them, EotS is supposed to be the Platinum to their D/P. I haven't played it myself but I hear it's excellent. But if you haven't played another MD game, you miiight want to hold off until it's cheaper because the series is definitely not for everyone. Some people love it but I know others who despise it.
2. Have you played the other KH games? If no, I wouldn't suggest 358/2 because it would probably be confusing as hell. If you have, go for it. It's not the most amazing story but it does have fun gameplay (it's the same kind as in KH1 and 2, so no cards, which most hated on CoM) and the plot bridges the gaps between all three games out so far. If you're a fan of the series, it's good to have, but otherwise, there are other games to get first.
If you answered yes to both of them, I'd probably just suggest KH because I really, really enjoyed 358/2 and prefer it over the PMD games that I've played. :p