+100 MP will obviously grant you a bonus to your pool.
+4 stats gets a little more interesting. 4 Int will transfer to 60 MP (1 Int = 15 MP) Thus giving you 40 MP less than the other enchant. Are you willing to sacrifice 40 MP for increased health, regen and so on? I know I am.
Oh yeah I wasn't totally sure on the Int -> Mana conversion, but if it's only 40 mana I think the stats are totally worth it. :] Thanks for the input!
EDIT: whoops sorry I'm trying to compare between +3 stats, not +4, which I think is objectively better. but I guess the break down with +3 stats would be...
+45 Mana (1 INT = 15 Mana) +30 Health (1 STAM = 10 Health) +1.665 MP5 (1SPI = 0.555 MP5 [for Druids])
I'd just go with +stats and call it day. You get more from stats than just mana. Also, the extra 1 stat you get from the +4 stats enchant isn't worth it unless you have lots of gold to afford it (I heard it was going for around 100g a couple months back) and you're flasking/doing progression in a top guild, which on this server is nobody...