Mountain Bike Hardtail Frames

I am 5’11″, is a 17.5″ or 18.5″ a better frame choice for this TREK Hardtail?
I have riden a TREK 17.5″ but not an 18.5. Both feel pretty good, but for some reason 17.5″ seems really small, not that it looks small, it just sounds too small. I am 215 Ibs. if this helps. Which is a better frame choice?
I am referring to the below bike.
http://www.trekbikes.com/us/en/bikes/mountain_hardtail/6_series/6700disc/
You should get the 18.5 inch the 17.5 will feel cramped at your height.
2009 K2 ZED 4.4 HARDTAIL MOUNTAIN BIKE

