Depends on whether you want driftwood or bogwood.
I have Mopani as i like the two tone effect, but i also have bogwood in another tank for some of my BN's. I don't like driftwood for the very reason it does just that... drift! I guess you could anchor it down somehow though.
As for harming the fish, i don't think the tanins released do any harm to fish, as said, mine is still staining the water now after 6 months and my fish are fine. It's just a pain if you like 'clean' water. I was annoyed at first, but now i just live with it. Quite like the natural look of it now.
Just a note, my mopani was about a fiver a piece where as driftwood and roots can be quite expensive.