Behind it would be favourite......it's a big lump of terrain - it's going to get destroyed, you might as well make sure it's somewhere worthwhilewen it does.
Ard boyz can take an exploding vehicle pretty well provided they're not inside when it happens.
I'd recommend red paint and maybe a turbo boosta - turn one get as far forward as you can without presenting yourself to heavy weapons, turn two move out, dump off the ardboyz and block fire to as much of your army as you can with the rhino. Put the ardboyz in front if you've got burnas to let rip with, behind if not...