sub job questions

[Calianeri]

the MP boost is far more than 30-40 MP

Nope. SMN sub gives about 3 MP per level of the sub. BLM gives about 2. (Race affects the exact numbers, these are estimates.) SMN "Max MP Boost" trait at level 10 is +10 MP.

So WHM40/BLM20 has about 40 MP from the subjob plus Conserve MP ability which can reduce MP cost of spells. WHM40/SMN20 has about 70 MP from the subjob. The difference is roughly 30 MP. That's a nice boost for /SMN, but /BLM's Conserve MP only has to kick in a couple times in an XP chain to make that up.

Also there is the cost of leveling a BLM to 25 v.s. SMN - with SMN you only need to buy level 20 equip and no spells

If you are basing your choice of sub on cost, that's too bad. It's not very expensive to keep BLM up-to-date, and there are certain spells you can skip if you have to. The best BLM spells (Warp, Aspire) are free! The other really useful spells are pretty cheap (level 1 elemental spells, Blind, Bind, Bio, Shock, Frost, Burn).

Before level 30, it probably doesn't really matter what you sub -- BLM, RDM, or SMN. Nothing huge one way or the other. But when you hit the next few milestones you'll find that BLM always has great stuff to offer with no conditions or caveats. I'm looking forward to trying out /SMN when I get some avatars... but until then it just doesn't have enough going for it.