Would you play a Class with an XP adjustment?

[Lord Pendragon]

3rd edition has had Level Adjusted races since the begining, based on the idea that you could play a more powerful race in exchange for progressing as a character more slowly.

Not quite. You can play a more powerful race in exchange for progressing in class more slowly. Your character progression is exactly the same as a character not playing a +LA race.

However, would you play a Class that was slightly more powerful than other classes if it meant you leveled up less frequently than your peers? Basically a class where you had an XP penalty of say 10% or 25% or even 50%.

The issue is, when would it be slightly more powerful? Would there ever be a time when it was not? Would the characters ever be balanced?

Thus say a 4th level Swordsman would be balanced against a 2nd level Mage, or the like.

How do they stack up at 8th/4th? At 16th/8th? Etc. etc. Are they always at balanced levels? i.e. does the xp progression work out so that the lower-level mage is always balanced with his higher-level fighter buddy? Or are there points where the mage is more powerful than the fighter in the same party, despite the fact that the fighter might have more levels? Or the reverse?

It just doesn't strike me as a good idea. If anything, I'd probably use the Ars Magica approach of using multiple characters and just have each player's "uber-character" have more levels. Having any kind of "more powerful" class is counter to the entire system.