Opacity maps vs geomerty for foliage

Hi guys,

I new to this forum and to unreal engine. I searched the forum but I couldn’t find a thread about this yet. (sorry if it is out there).

I want to create my own patches of grass for architectural visualization and I was wondering what strategy works best.
I work with Corona render and Vray in 3ds max to render stills and there have been tests that show that opacity maps are way slower as using a higher amount of geometry.
So are cards, with in this case, grass leaves with opacity maps, slower as a larger amount of single leaves without opacity maps? Do opacity maps affect the speed as they do in a renderer?
Of course UE4 works different as Corona Render/Vray (GPU vs CPU) but still I was wondering if anyone has tested this.
Or is there some golden rule that says what a good ratio is? Like if I add for instance 10 vertices to a leaf or card with leaves I need to use less opacity map area.
In this case I need more geometry to process but the reduction of opacity speed things up. Or is this interpretation wrong?

Thanks for your replies!

ps. I hope I haven’t asked a really stupid question btw.