I liked your initial 21/15 build.
It knew what it wanted to do (infantry support), and is good at it. The new one (21/18) is trying to be a lancer/ground-pounder/infantry support unit while trying to be viable. Jack of all trades, master of none.
I'd advocate the original build, because at 3 ironflesh (with the new build), you're not surviving any kind of dehorsing. Remember the circumstances of your rider. What kind of scenario are you basing the build off ? Will you be near teammates most of the time ? Will you be waist deep in enemy axes and swords when your horse goes down?
Building you character is fine and jolly, but outside the lab - in the battlefield - it's a very different game.
edit: my build is an infantry support build, somewhat similar to your initial proposed build. Capable on the ground, and able to ride if my team needs a sweeper. That's it. I don't try to shoot, I don't try to hit and run, I don't try to do what I cannot (unless we're scrimming).
I run defense for my infantry teammates, and take down anything that is an immediate threat to my teammates' (flanking cav, agility builds etc). Your new build will be more for lancing (hit and run, trying to couch the spawners etc), you will find that your combat potential on the ground will be hampered because of the combination of your low athletics and ironflesh.