Candidate one: 11 y.o. TB mare, former eventer. Very responsive to aids--if I wiggle a finger incorrectly, she's going to make it clear that I screwed up. Gorgeous looker, attitude up the wazoo. Beautiful sitting trot, though getting forward without having her hollow her back is a struggle (that's me, not her). An old injury means that she canters quite oddly--sort of corkscrews her left hock leg under her, so her flying lead changes are...interesting. We get along well, but she's a mare through and through.
Candidate two: 8 y.o. OTTB. When you ride him, you WORK. We've been tackling the basics: keeping a bend on a circle, keeping him forward and on the bit, turning his giant canter into something resembling a collected gait, etc. He's a big sweet scaredy-cat. Most students don't handle his spooking well, but he and I have a rapport and I don't escalate his flight/fight responses.
Here's why I'm torn: the mare is more responsive, but the gelding really makes me use every muscle I have. At this point, I feel like the biggest things holding me back are my (not so strong) leg muscles and my too-forward seat. I know there's really no easy way to choose a horse without seeing the rider on it, but if you have any thoughts at all I'd be grateful. I asked my instructor, too, but she feels that both horses would really benefit from the dressage work (they're not usually used for that, as our barn is more hunter focused). Like I said, I know there's nothing concrete you can probably say - but if you were still early in your learning, would you want a horse that's more sensitive, or a horse that asks a bit more of you up front? They are both willing to learn (and tolerate me :)).