Hi Mark-
My $.02...since you asked. I am 5' 10". I test rode the 17" ST2. it was clearly too small for me. I then test rode the 20", I'm very comfortable on it. So...I bought the 20" ST2.
My take, a good deal isn't a good deal if it doesn't fit you. The price may be right but you may cause yourself knee problems and all the time you are riding it you will wish you had a proper size bike....and....when that good deal for 20" comes along you will have already spent your money.
Best Wishes!
-Steven