depends on the cut and tightness. you can get rashguards which are loose fitting, i.e. sprawl ones, which breathe alot easier cause they arent hugging your figure. (but by doing so are not tight like compression gear, obviously), and in the sense it is just a shirt, thats loose, and wont tear if you grapple in it.
normal rashys, which have a tighter cut, but arent always 'right on you'
and then hybrid compression gear which can be a bit thinner than a rashy material but it's tight on you, it hugs the skin and increases blood flow by thus helping your muscles. or so i was told. my dad wears 'Skinz' it's a compression gear stuff..
~Foz