Most of my MF stable is TLRs and folders, and while I like both, they're very different animals. A well-made TLR may have more of a "normal camera" feeling to someone coming from modern 35mm gear; it basically functions like an SLR with bizarre ergonomics and no mirror blackout, whereas a folder might have more of a "weird old machinery" feel. All things considered, I'd look to TLRs first for someone new to MF.
Some of the 645 SLRs are relatively cheap and have more of a giant-version-of-35mm working style. My Mamiya 645 has basically put my 35mm gear out of a job for most purposes. Even a small MF SLR is pretty enormous compared to 35mm gear, though, and while they don't *look* much bigger than TLRs, gravity sez different!