Hello robbm,
Unless you need to practice your cursing, I would not recommend THHN. Control cabinets wired with THHN are very difficult to work in. When the wireway covers are removed and the wire disturb, THHN tends to "eject" itself from inside the wireway(and I do mean eject). This leaves the cabinet in a mess and makes tracing and replacing the wiring far more troublesome.
The only other difference I can think of that has not been mentioned above is the temperature rating. THHN is rated for 90* C, while MTW is available in 60* or 90* ratings.
Bigbillnky,C.E.F.....(Chief Electrical Flunky)