Ride them all, but I think the 17.5" size might be a little on the small size. I'm 5'9 and ride a 19" hardtail and its about the right size. I bought a 17" frame and it felt to small.
I'm a commuter, so my likings might be different. A guy at my LBS offered to buy the 19" bike and he is about 6' and actually rode trails and stuff.
Without knowing your area, are there any coops or LBS that sell used. I bought my hardtail (
Jamis exile) for about $350 from a LBS that sell used. I bought it around 2006 and it was a 1999 model that retailed for about $650. With the exception of a few scuff marks everything was stock and in excellent condition. They basically said the guy bought it around the beginning of summer and returned it at the end and only rode it a few times.