I bought a sit-down (Bernina Q20) and I have zero regrets! A sit-down is a very different animal than a stand-up LA, in my opinion. Ideally, I'd have both!
I shopped for mine for 2 years, test-driving every machine I could get my hands on - I was 100% certain by the time I made my purchase that I was buying what I really wanted. If you're thinking you might change your mind, maybe you're not quite done shopping?
Have you made "pro" and "con" lists for each type of machine? Do you have a really clear idea of what type of projects you'll be doing on the machine? Have you spent a solid amount of time working on each type of machine?
For a LA, I discovered that my back problems will be an issue if I ever get one. I think I can make do with having a stool to perch on and perhaps by wearing a back brace, but definitely I found a sit-down machine far more comfortable and can spend hours at a time working on it. If I ever do get a stand-up LA I will need to make sure it's one that glides as effortlessly as possible, to avoid fatiguing or tweaking my spine. Depending on your age and health, that may be something to take into consideration too, if you haven't already.