Well, it's really a tough choice for me because they're both amazing companies which make the best roller coasters in the world, in my opinion. Looking at my top 5:
Top Thrill Dragster- Intamin
Millennium Force- Intamin
Magnum- Arrow
Raptor- B&M
Wicked Twister- Intamin
So it seems that Intamin dominates my top 5, but I really still can't choose because I've been on quite a few Intamins in comparison to the amount of B&M's I have been on.
In the long run, if I had to pick one I think I would pick Intamin, just because most of the coasters I think look like the best in the world right now are made by them (Kingda, Dragster, El Toro, S:RoS, etc), and the record breaker is also Intamin.
B&M coasters are more varied though, I believe. You have flyers, Inverts, Sit Down, Floorless, Hyper and Stand up, just to name a few. Now though this is good, the layouts normally follow the same pattern, for Stand Ups, Sit Downs, Floorless and Inverted. Intamin, which makes mostly Hypers it seems, usually have quite varied layouts, despite being the same kind of ride over and over again if you get what I mean.
Looking at it on a world standpoint, on Mitch Hawkers poll the top 3 coasters are all Intamins. That's taking into consideration the whole world's opinion, and not just mine.
Thus, I think I am going to have to pick Intamin, even though B&M is a really close second.