18/21 seems like the best balance between power and mobility. Both are pretty important depending on the situation you are in so a balance is good. For instance with the 15/24 you really can't do to much in a tight area, you need space for that build to really shine, so strat sieges (which should increase in frequency now) would be a real bitch. And then with the 21/18 you probably are going to find yourself outpaced by the increase in agi builds during field battles (and when pubbing). That said 21/18 would do better in sieges, so if you play siege when pubbing, and want to play in a lot of the strat sieges that should come about do to the removal of the 1/3 rule, you might find it a bit more suitable.
I myself have played as a shielder for most of my gens now (packet loss makes consistently manual blocking difficult mkay) and I find that 18/21 really gives you the most versatility so long as you are going to level 31 at least. Even so it is going to be a different ball game now with the way agi is, so you might go with 15/24 just to keep up with the new agi builds for awhile. Then assuming your not going beyond lvl 33 you can retire to 18/21 once the agi craze has settled down a little.
Now if you are planning on going beyond lvl 33 I would recommend checking out 21/21 or 18/24 (unless you want lots of riding, lets say).
and of course the most important thing is to do what enjoy the most, so basically don't really listen to me.
At lvl 32 I had this (with old wpf ofc):
Level: 32
Health Points: 53
Weapon Proficiency Points: 2
Strength: 18
Agility: 24
One Handed: 149
Power Strike: 6
Shield: 4
Athletics: 8
Weapon Master: 5
better than 15-24 according to me.