Ignoring all mental attributes, who makes a better rigger? The troll may not fit in most vehicles, but is that an issue if he's in a van? He can more easily work on vehicles and larger drones, changing out weapons and loading up missiles. On the other hand, orks get into areas trolls can't.

So what do you think?

Go for Ork: Strenght is only a little bit lower compared to the size-difference. Body-difference is even less significant. Like you already pointed out, trolls are quite noticeable, they are way too big and also too heavy. Being a troll is cool in close combat (reach, higher strenght, natural armor), but think about it realistically....it is a lot of trouble in your everydays life.