Here's my thoughts about Pilates, especially given Knott's opinion about your spine.

Not-very-expert Pilates instructors aren't well trained in recognizing when their participants are in neutral spine and when they aren't. They'll tend to put participants in imprint/C-curve more than they ought to, because when your legs are in the air, it's a lot easier to stabilize the spine if you flatten it against the mat, and you definitely don't want your spine flopping around when you're doing the movements. So rather than closely observe the participants and limit each one to what she can do in neutral, the instructors will put everyone in imprint and let them flail away.

Add the fact that classical Pilates has a lot more exercises in spine flexion than extension. Add in an aging pool of group fitness instructors who mostly came from the aerobics boom of the 80s when everything was "suck and tuck." Add in that most people these days are sedentary outside of their "exercise" activities, sitting in chairs with their pelvises tilted posteriorly and their lumbar spines collapsed back. What you get (what *I* got) is a severely flattened lumbar spine. I see it in lots and lots of people now that I'm learning to know what I'm looking at.

That's why yoga has been so wonderfully exactly what I've needed. Seriously, I'm getting better, but I really have to use some muscle to get my spine into neutral, and it "feels" like I'm putting a huge arch in my back. But I really can't generalize from my experience, because just as I've pretty much only done Pilates with poorly trained teachers and/or from videos, I've pretty much only done yoga with amazingly good teachers. So I can't say that with equally trained teachers, one is better than the other.

But I do think that when you have a potential undiagnosed spine misalignment, a gym Pilates class is a bad idea. Find out what certifications the teachers have. Sometimes you do get a really well trained teacher in a gym - someone who doesn't need to get a return on their investment and just wants to spread the love. But it's pretty unusual.