Since Persona technically isn't SMT, I might as well say I've never played an SMT game. I've also only played the original Luigi's Mansion for like 5 minutes, back when the Gamecube was new. I've heard Dark Moon is a bit on the short side, so I dunno. Can't decide. I'm leaning towards Soul Hackers though.
Persona is more like a barebones SMT, so it's not like you haven't exactly played SMT. Soul Hackers is a bit more similar to the classic SMT design. However, Soul Hackers is a tad bit outdated, and there are a few things that the game excels at, but there are a lot of things that it could have done better in a modern perspective.
I'd say go for Luigi's Mansion and wait for SMT IV like others have said.
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