I have flown several Bob Parkinson Regal Eagles, a Byron F-16 and a JMP Starfire off of grass. I have never run a Ramtec, but have flown the Byron and the Dynamax. My choice is Dynamax hands down.

Grass is going to limit the number of models you'll be able to fly. Heavily loaded scale airframes with small tires should be avoided. You will need to be more concerned with weight than you would if flying off of pavement. You will also need a slightly more nose high stance, to help rotation from grass.

Here is a project (Electric F-4 Phantom) that I would have never thought would be successful off of grass. Be sure and watch the video.
http://www.rcuniverse.com/forum/m_32...tm.htm#3343697