Well, the heavy bolter is an anti-light infantry weapon, and it pairs great with the hull heavy bolter or heavy flamer, and also with stubbers, if that's how you roll. Multilasers, on the other hand, are anti-
AV-10 weapons without really any other purpose.
Multilasers are technically better against marines than are heavy bolters, but that doesn't make them GOOD against marines, it just makes them less bad. As such, I'd personally take the slight hit and keep the heavy bolter, because you'll be glad that you kept it once you get around to fighting a non-marine army.
I used to run with a chimera in 2x
HB mode with stubber with a squad with a
GL+
HB inside. I guarantee that setup will make your opponent cry if they get their light infantry caught in the open. The multilaser, on the other hand, can patter helplessly off of flak armor (which is really, really annoying when it happens), and creates a mixed-role model, which is silly given the massive specialization opportunities of the imperial guard as a whole.
Of course, with regards to modelling, there are ways to make that turret heavy bolter better looking. Either chop it back a bit (lot), and angle it down, or redo it with something more interesting like my old 4x machine gun boat: