right off the bat i'd tell you to get a 20". i'm 5'9" too and will buy a 20" 7.2 fx tomorrow. just like you, i have long legs for my height. i have a 33.5" cycling inseam. these days its fashionable to buy bikes that are too small but don't fall for it.

if you buy a bike that is too small you're going to have a ton of seat post showing and the handlebars are going to be low. that's a recipe for back pain.

hop on a bike, pedal backwards with your heel, and keep raising the saddle until there is no bend in your knee when you do a full backward stroke. now you've got a rough idea of saddle height. then check to make sure that the seat post is not higher than 7-9". i personally make sure that handlebar is even if not higher than my saddle at proper height. it's my preference for comfort.

the above are pointers on what to look for when doing a test ride. there is absolutely no substitute (and better way) than to go to a shop, do a test ride, see how you feel and get professional feedback. some shops are dishonest and they just want to get rid of whatever they have...so go to several if you are unsure. don't go based on what others think are comfortable or what they ride...they can be same height yet very different in proportions...they may have different riding styles.