I've found that Warp (The black hole one, I think) seems to be more acurate, when used in battle. Warp, on the field, teleports you back a number of floors equal to the level. E.G, a level 11 Warp spell will take you back 11 floors. Exit does not seem to have any added effect from levelling up, other than costing more MP, and slightly increased Accuracy in battle. Once you level up Warp to, say level 8, you'll never need to use Exit again, as no dungeon is more than 16 floors. A Level 16 Warp spell is basically a Level 16 Exit without the HP Drain, and it's also more accurate.

This topic is for Dawn of Souls im guessing, as Warp and Exit are known as Banish and Warp in Origins. I'm not sure about the originals, you didn't say, Peter. Be a bit more clear next time, okay?

Hope this info helps.