As long as your trees have SpeedTree kind of optimization you should be fine(as fine as having foliage inside your level goes. Large scale foliage always has a cost and balancing quality&performance is up to you) If you could model each leaf and used opaque material for them then you would be having trouble with poly count. You’ll have issues with both technique when it comes to large scale foliage. So making sure your trees have proper LOD’s and billboards is more important really. Epic also has GDC talk on how they handled trees that are far away from the camera:
https://www.youtube.com/watch?v=hOMjp6H10Mg