My main objection with the heavy bolter one is that it's just worse than a rhino. The squad inside can fire more than it can if they open up the top hatch. If you wanted your army to have a kill point with a single heavy bolter, which is possible I suppose, you could get an attack bike.
I find the tactical applications for razorbacks to be pretty limited. I have had some success with a unit of 6 grey hunters (5 and a WG) in a razorback with las/plas. This unit can loiter around behind my main assault, which tends to involve land raiders, and help to pick off things like transports or obliterators.
While there is a trend currently for meltas and long fangs, I tend to appreciate things that can fire on turn one, which means mobile long ranged things like typhoons and razorbacks. A shot on turn 1 is a lot more valuable than a shot on turn 2 or 3, because it has much more of a destabalising effect on your opponent to break his transports in his deployment zone. A razorback with a lascannon and a squad inside can really help to win the game, not by doing anything spectacular but by helping out where needed and keeping a useful reserve scoring unit to grab objectives late in the game.