Yeah, transmaps vs. full geometry on plants is definitely a trade off. IMO, it depends on where you need the boost.
If you need the boost during scene setup, transmapped plants are great for filling out backgrounds, like forests, fields, plains, mountains, etc.
But if you need the boost during rendering, then full geometry is definitely preferable.
For me, since I don't do Iray
(I only render in 3Delight), and I have, as some have pointed out, a tendency to build complicated or at least very detail oriented scenes with lots of bits and pieces... well, I need the boost in the scene setup. I don't mind setting a DOF render to run and then going to bed while it does it's thing. I get up about 4-5 hours later and the render is done.
Iray takes my machine 24-36 hours to render even a VERY empty, simple, basic scene... so.. yeah I don't have the patience for Iray. It does gorgeous work, but the tradeoff on render times is a game over move for me.
It's like anything else in the hobby. All depends on what we need and where we need the boost or memory to go.
