In my opinion, Intamin is the better company. I may be a bit influenced, though as i have two examples of Intamin's greatness next door (Expedition GeForce and Colossos) while Black Mamba and especially Silver Star were kind of boring. On Silver Star you feel just like riding a cabrio on a hilly highway and you can easily just sit there, enjoy the view and have some small talk with your neighbor. On EGF, hovever you just enjoy the ride and the totally awesome ejector airtime while your eyes are watering due to the wind - on every row!
When I look at the recent developments in the amusement industry, I notice that B&M cares more for a safe and reliable ride rather than a totally awesome one like Intamin. And although B&M seems to be pushing the limits a bit more in the last years (crazy chinese flyer, Leviathan), Intamin is just even more extreme"OMFG" experience rather than a smooth hilly trip that might or might not include one inversion or two.
Also I believe that the downtimes of typical Intamin rides are a bit exaggerated - in the last years I have visited Holiday Park and Heide Park quite often and there were never any problems with their Intamin rides - be it a coaster with a lift hill, a launch coaster (Desert Race), a Free Fall Tower or anything else. However, when I went to Europa Park, Silver Star broke down for half an hour while I was in line. That may not say anything, though.
What I can't understand about Intamin however is their restraint policy. I'm quite tall (1,97m/6ft 5inches) and I don't like Intamin's OTSRs at all- to me, they feel amost as restricting as a "standard" OTSR. I think it is a shame that a classic Giga coaster like i305 has them (although it may be necessary due to the massive amount of extrme twists on the ride) and especially a ride like Skyrush would be 10 times better without them, I believe. Apart from this, however I can't understand why there has been only one major Intamin coaster announced for 2012 while B&M will build 6 or 7.