Hmm....I'd have to go with voxels on this one for sure....Like Novalogic's Delta Force 2. Voxels are not as computationally intesive as polygons so you shouldn't notice TOO much of a FPS drop.I could be wrong.It all depends on what area you want to cover with grass.
I would suggest using Voxels for the grass but only reneder the grass if it's a certain distance away from the player.That's what Novalogic did.