Check the item guide for that. But to be honest, I'd keep Charti. 80 BP is already nice and it has a nice effect. Yes, Micle is 20 BP more, but can screw you over by being consumed when you don't want it to be. Micle actovates all the time when at low health (which will be often, Tropius isn't all that bulky, tbh),so once yiu get low, any Harvested Micle berry will imediately be consumed again, making it so you can't go for Natural Gift. Wiki is similar, heals but confuses, so that's bad too.
On the other hand, the majority of things that use Rock moves are things you wouldn't attack woth NG Rock anyways (griund, rock, foghting, etc).. So yeah, I'd say go with Charti. Magost would maybe be handy simce it has no effect and is 10 BP more, for when you know the foe's got no Rock moves, but it's just 10 BP more...and I don't thinl we have that anyways.