Aggie, I haven't yet tried trail riding but my friends who do say if you can only afford one go with a great helmet mount, but for any real technical riding and/or trying to maintain speed it's really really worth it to go with a bar mount AND a helmet mount.

With two you can still "look where you want to go" with the helmet mount but the bar mount illuminates the trail where you're at.

I only commute on the road, and have been very happy with my Light & Motion Solo Logic MV. Also not cheap, but the customer support has been awesome over the years and even though they don't make that particular light I can still easily find replacement bulbs and other parts. I hear they and NiteRider are almost always found supporting the big 24 hour races.