1. Regain Roll.
Evoker's Roll caps at 5 with an 11 w/ SMN in party. Realistically ~3 is to be expected. Regain is always rarer and lower than Refresh (drinks, atmas, lack of it on gear, etc), so I think it's a safe assumption to say it'll be lower here. Even calling it 3tp/tick...
Sam roll w/ Sam in party is pretty safe to assume ~30 stp. For a Sam that's 11.1*.3=3.33 per hit (which is <1 tick). For a Mnk that's 3.7 avg per attack around, again faster than 1 tick.
In addition to this, you can rely on StoreTP, it -will- coincide with your attack round. On the other hand, regain can tick at inopportune times and is a pain to plan for.
Really, I don't see a regain roll beating Sam roll in a realistic situation...it will likely be lower than what I said, as I'm comparing to a smn in party evoker's roll, and I think I'm overrating regain. That being said, it's nice for "pre buffing"...enter a BC and put it up while mages buff/rest MP or something.
2. /sch vs /rdm.
Giving up gravity, 5% fast cast, refresh, convert, MDB2, Phalanx, slow, paralyze, silence.
Gain 2.5% recast, -10% mp cost, 2 strategems, Sublimation...can -na/erase, but it's a huge sacrifice, and /whm is honestly better if you think you'll need them...
Honestly, not going to do all the math, but I don't think it's anywhere near the tradeoff people are making it out to be. It's lackluster at best, and likely worse for you overall. Definitely bad for small mobs where silence/grav are nice. Slow and para are nice to have, and really, you'd need to be looking at an ~8-9min fight for it to even compete on MP. Otherwise convert front-loads MP.
3. Sch/nin solo/duo.
Still missing out on 45% fastcast for ichi, slow2, a useful 2hr, unlimited sustainable mp, haste, addle, gravity, bind, silence(?), good haste gear, good pdt/mdt gear, relies on grimoire. Is it much better than before? For sure. Is it a red mage? Most definitely not.