Honestly, same...^^; I've never heard either of those pronunciations before...
Maybe it's a Canadian thing; here in the States it's usually MAN-ga vs. MON-ga...with most people accepting MON-ga as the "correct" pronunciation, simply because it adheres to American English's unspoken rules about how 'foreign words' should be pronounced...not because it actually sounds anywhere near accurate.
I've always found it really unnecessary to deform Japanese words like that when we already have similar vowels in English (in this case, a short 'a') that are totally OK to use. As someone who can understand spoken Japanese to a degree; it's a lot easier to understand someone simply speaking Japanese with an American accent than to try and decipher the "Japanglish" that most anime fans use...