I'll start by thanking everyone who suggested I go test ride a couple of other horses- it was such a valuable experience.
Today I rode the little grey mare, plus a 13 yr-old paint gelding who was a little bigger than I thought I was interested in, but also ended up being a nice ride. The two were just as they had been advertised- quiet, good mind, willing to please, good ground manners. I had an hour "lesson" on each horse, and then spent another hour riding out on the barn's trails. The BO was so generous with her time and asked lots of great questions to try to be sure I was articulating what I was looking for so she could help make a good match.
The whole time, I will say I just wasn't feeling great about my riding- I felt a little out of synch with each of the horses, never really felt balanced, and just overall didn't feel like I was getting them to perform their best. It was clear both horses have more experience with Western/trail riding- they didn't really like being ridden with contact, and didn't really have a lot of experience with things like changing on the diagonal, serpentines, figure 8's, etc. That's fine, I know it's small stuff that can be fixed with work. It just didn't feel like my best riding since what I'm used to and what they were used to didn't necessarily mesh right away.
But the thing is, that's also exactly how my lease mare was 2 years ago when I started working with her, and know she finally understands those things. Do I want to go through that all over again with a new horse when lease mare is finally working pretty well? I also realized that I just feel really comfortable on her. I realized that what I'm seeking, more than any particular kind of riding (trails vs. arena, English vs. Western, etc.), is just that feeling of being comfortable, relaxed, and predictable. I realized that I want riding to be something where I know what to expect, and am not constantly striving to move up a scale of ability. I work in a really intense environment and have always been an overachiever in my professional life- I kind of want my riding to be the opposite of that, where I don't have a project I'm working on improving, but just have a horse where what I get is pretty clear and upfront- not a lot of surprises. The lease mare is all those things- I know what she can do, I know what would be harder for her. But I also realized today that I really know how to ride her, and I'm a much better rider on her. For all those reasons, I think I'm really leaning toward sticking with the lease mare.
I've decided I'm going to have a vet check on the lease mare on Thursday and will have a really honest conversation about her back and any future limitations that might limit her rideability in the near-term (i.e., the next 2-5 years). I know a vet can't guarantee anything, but I want to feel that I'm making an informed decision. If the vet check doesn't go well, I suppose I will need to regroup and continue the search.
I do want to thank everyone who gave such good advice and was so encouraging, and also acknowledge that this decision is probably out of step with what most of you would have recommended. The thing is, I just can't shake the feeling of wanting the comfortable, broken in pair of jeans vs. a newer, fancier version that will take a lot of wearing in. I may not always be in that place with my riding, but right now, that's my honest assessment of what I want, so I feel pretty at peace with this.