Raiden or dead moon? I've played them both in the past and I enjoy them both but I can only afford one at the moment(i'm also buying a turbo booster and pc engine duo) so some opinions would be nice.
Raiden is a classic vertical shooter. It's tough as balls, so unless you really want to dedicate some time to it, you might be frustrated. Also, it is available on many platforms (though the TG-16 version is friggin' awesome). Raiden is awesome. Oh, I said that already.
Dead Moon is not available on any other platform (as far as I know, anyway... it might be on some Japanese computer platform) -- so it's novelty (a relatively unknown shooter) that intrigues some folks (like me, a slut for any shooter). Dead Moon is an easier game, so it might be more fun to get in to. While it has been critisized for being a "generic" shooter, folks here have praised it recently (I think nodtveidt started it off). I think it is a average shooter, myself, but I know I had fun playing it. (Keep in mind that I am biased towards old-skool shooters).
There is nothing wrong with average shooters on the PCE / TG-16. They have stiff competition on this console. I'm going to buck the trend and say that it's a toss-up between the two. I don't know how expensive (or cheap) these games are on ebay... so go with the one that fits your budget.
NOTICE for DIE HARD SHOOTER FANS: There is a CD version of Raiden! If you are a hardcore shooter fan, you might want to hold out for the CD version, though I think it might be somewhat expensive on ebay.