I have a frame but have only used it for practice so I don't have anything to add but just wanted to toss out some questions/thoughts in case you haven't thought about them.
The frame I have is adjustable in length from crib to king size so I have it at queen but have the ability to add the extensions if I were to need them. Some frames are supposed to be collapsable so they can be stored when not in use.
Standing ... some people use a saddle stool to roll along as they quilt on a frame
With a sit down frame you would still have to baste the quilt (I can't crawl around on the floor ... bad knees)
With a frame your quilt orientation stays the same where with a sit down frame you can turn it every which way (a plus in my book because I'm not good enought to draw upside down)
hmm, that's all I can think of for now. Good luck in your search and I'd like to hear what you decide and what you think after you've tried it. The frame I have was given to me and I'm not entirely convinced that I wouldn't like a sit-down system as well.
Susan