For training magic, I'd say there are two excellent places. In the World of Ruin, the fanatics tower, bar none. On the other hand, if you don't have a lot of magic already, its kinda crappy. Then in the World of Balance, Triangle Island, fighting Intigir's (I think thats what they're called) and casting doom on them is the best way to go. 10 MP per battle. The only trouble is that you'll run into a lot of weak pathetic thingies before finding one. But not so much that it makes it impossible, just enough to make it reasonable.

If you're trying to get experience, go anywhere, and just work your way up to a million GP. I got a party up to the 70's by doing that. I just wanted to get the Model Blackjack from the Auction House. That $*&! little kid and his &#@* rich parent... Well, a million GP is a high price to meet, so by the time you get there, you'll be wealthy in exp as well. Provided you didn't go to the Veldt (which is pretty random, so you probably shouldn't anyway)