You have to ride each size of the bike and see which one is more comfortable. It depends on a lot of factors, every bike will fit everyone differently. I'm basically your size (6'1" and 190) and my 21" Spec RH is a perfect fit FOR ME.
I ride a HardRock too, 6-2, 215, in 21/xL, and my other mtb is a Marin Hawk Hill at 18.5". Different geometry between them, so ride as many brands as you can in different sizes. If possible, take it for more than the cursory ride around the block to see what feels good. As with most sports, it's all in the fit.
Depends on what and how you ride. Most will say that the smaller bike will provide better handling but you don't want to be cramped in the cockpit. I'm 6'4 and have a 21" Schwinn hard tail from 2000 and a 2005 XL SC Heckler (very different geometry but similar overall size). Any smaller and I'd feel claustrophobic.