It depends. For me (graduating University with a degree in Computer Engineering this year), a flash card is much more enjoyable than any one game because you can really get your hands dirty with it. I own a 3DS and no 3DS games, just a DS flash cart. It lets me play all of my favorite games from my childhood (including the original Pokemon games) using software made by members here at this site, play music, customize it with themes, etc. It also lets me carry all of the DS games I own with me at all times, because they all fit on the same card. It also opens up the door for savegame modding and all kinds of other fun stuff.
Obviously, you really have to be that kind of person to be into that. Luigi's Mansion is supposedly a very good game, and from the little I dabbled in it, it has excellent graphics, so I'm sure you'd really enjoy that as well. Plus, nothing beats playing a game with a new and active community around it. You'd also be supporting Nintendo and helping to encourage future awesome titles on the console (and everyone wants that, right?).
It really comes down to what you want out of your console.