THM does get more HP (I think I notice about 200+ HP when going from BLM to THM).
THM also gets a lot more 'sustainability' skills to keep them going. And even some additional magicks.

However, they then loose their BLM specific skills. Most notably:
Sleepga (super great for crowd control).
Convert/Freeze (if they are in an all-out-non-stop-moogle-nuke-party).

If you do not need crowd control, and you are not in a situation where you need or can nuke non-stop till MP is depleted, then you may have hit a situation where BL Mis non-beneficial.