acumen is the best sa for any mage that plans on casting anything, ever. Possibly with a mana up weapon you can switch to to gain a few more mp form herbs. But as a main weapon. No.
You best weapon should be Acumen. Then get a Crystal Staff for Mana Up when needed (like raids) and a +0 Homu with Conversion for when you really need a lot of mana.
My SE has all 3 SAs. My EE does also. My OL has Acumen and Mana Up.
I also have Tallum, Majestic, and DC Robe Sets to use on them if I play one actively, as I do the OL for some things (depending on what I'm doing).
If you can't afford multiple weapons (the low ones are very cheap though), Acumen should be the SA on you weapon, for reasons stated (casting Speed and Reuse).
Next update, it will help to have at least a Mana Up weapon for PvE reasons because the Heal Casting Speed is getting decreased and the PC castin speed won't affect skill reuse :)
I agree, Acumen for most situations, Mana Up to help. I don't have a Conversion weapon, but I assume it can help too.
Acumen sounds obvious because you're faster to save your friends and some skills cooldown are affected by casting speed.
Mana Up is trickier to use. It nice to have it at the beginning of a party to make the first buffs (Prayer+BOA are particularily MP eating, Divine Protection too), switch to Acumen weapon and see how's your party handling the spot. It gives you like a damper for your MP consumption the time your party takes a room and adjust to the spawn.
Then if your MP consumption is low enough to quickly get back to max MP, you won't suffer to buff with your Acumen weapon. If the consumption is constantly high, you won't get back to max MP anyway so Mana Up isn't useful. Maybe to be able to do Invocation before.
You can keep your Mana Up weapon if the consumption is not constant, if you can go back to max MP but you prefer to have a comfortable pool of MP in case the mobs get mad.
Also, if Concentration Dance is always on, you can generaly keep your Mana Up weapon.
In PvP, Mana Up to buff first time (for a gank), then Acumen all the way.
Next update, it will help to have at least a Mana Up weapon for PvE reasons because the Heal Casting Speed is getting decreased and the PC castin speed won't affect skill reuse :)
But if you are getting hit (Turn Undead or "oh sh..." situations in pole parties or whatever), still prefer Acumen (less interruptions, better ability to kite).