Cave Story is a very good and fun game. But it's a LOT shorter, and IMO, overall not as fun as SMT: Overclocked. (this is subjective, since they're both completely different).
Your first playthrough of SMT
C will take you quite a while, and assuming you do an ending with an 8th day (not every ending has one), there's even more to do.
I had it in my 3DS for a month straight, and only stopped playing it for a change of pace, and not because I was anywhere near done with it.
The game is pretty huge, and incredibly fun. There's a ton of branches you can take, different people to help/save/etc.
Even something as simple as collecting every demon so you can read their lore is fun (every demon/angel has a piece of lore so you know where it comes from, and it's pretty awesome).