I'll try to answer everyone's questions!
First, I have ridden both. The mare and I have ridden together many times and have a good bond; the gelding and I haven't ridden together as much, but he's a sweetheart (and I completely agree with the poster who mentioned making him bombproof in general. He needs some confidence boosting!)
The mare has a weak stifle, or at least that was the initial diagnosis awhile back. I'd love to have a vet out to see her, but we're both at the mercy of our barn owner who pays the bills. Since I'm not interested in competing, I don't mind the way the gait looks as much as I want to be sure that I'm not hurting her when I ride. We'd definitely do a lot of stifle strengthening work. There's a nice hill on the property for walks, and we also can build up to lots of leg yields, reinbacks and work over cavalettis at the w/t.
I actually decided to go with the mare. With Dutch, I know when I'm doing a proper leg yield or shoulder in because she responds; when my aids are off, she comes off the bit until I fix them. At this point, I want to have a stronger seat and better instinctual responses; I need a horse that can teach me to reach the next level. I learn a lot every time I ride her. To develop his own form and confidence, King needs more than I can offer him at this point. King will be a wonderful prospect when I'm ready to teach the horse. Poor guy hasn't had a half leaser in the history of his time at the barn, so he'll probably be waiting for me.
I'll let you know how things go. Thanks again, everyone! It is so very appreciated.