My impression is that com-rad is associated with the USA and com-raid is associated with Britain and Ireland. I'm not sure what the situation is for other English-speaking countries, but if I were to hazard a guess, one pronunciation over the other might be associated with the relative prestige or influence of British and American English for that person, for instance if you learned the word through American media you might pronounce it in an American way. I'm not an expert, though.
I personally say com-rad.