Well the honest answer is to offer the end user both.
However if the issue is related to performance then other things needs to be taken into consideration. Either case the foliage tool should be used so even small patches of grass could be useful as to inclusion so an assortment of unique patches would be more useful as the end user could then paint the required area.
To consider.
First concept the issue of moving and rendering geometry has been solved a long time ago so poly counts are not as relevant as to what they inherent from other assets and resources.
Shader complexity for example can have a higher performance curve than say raw geometry as to the use of typical transparency lighting solutions. Draw counts go up along with overdraw requirements that leads to complex shader issues. (you can test for complexity issues by using “render shader complexity”) If you see a lot of red or even white your going to have shader issue as to performance inherited by your geometry.
I mentioned this as being more critical as you are making a package to sell and the focus should be on what you can add that makes your asset easy to use, and in turn saves time, so the use of LOD elements that includes materials is must these days.