I use either a Fenix LD20 R4 (2xAA), or a 4sevens Quark turbo R5 (2xAA), in combination with a bike light I have. I use twofish lock blocks, they work great. I am commuting rather than doing offroad mtb. But its still pretty bumpy out there.
The LD20 is much floodier than the quark turbo. If I was only going to have one light I would pick the Quark turbo because it has about the right about of flood and throw for my needs. Best of all is having several lights of course
That way I aim one at the ground closer in (with the spot covering about 5-10 meters away) and the other almost horizontal. Having 2 different coloured lights helps in "being seen" by motorists, IMO over having 2 identical lights. My ld20 is a bit blu-ish, and the quark is a nice white.
I wouldn't be concerned about them being damaged on the bike by bumps. I haven't had any problems yet, they are pretty tough and there's nothing in them to damage like an incandescent bulb. I was more worried about the batteries being bumped around and degrading faster, but I have been tracking those on a maha c9000 and they are still at 100% capacity 9 months later cycling 125km a week.
If you are doing serious offroad mtb in the dark, I don't think a single 200 lumen light would be enough (well, at least not for me!), I'd want at least 2 with one on my helmet so I can see where I look, not just straight ahead. I would personally have the throwier light on my helmet too.