A couple of years ago I bought a shed load of 15mm Aliens M577 APC's from Anteonciti's Workshop. These are no longer available but, as I have a full company worth of all of the variants, that's not a problem (they had a special offer...what can I say?). The models are really very good and will be used alongside GZG NAC infantry, which are as close as I could get to Colonial Marines.
In the end, I've decided to go for the Warmonger heavy tank from Critical Mass Games, although it doesn't look at all like the designs in the book. It's a much more angular design with a conventional turret mounting but I think it will look good next to the M577's, which are similar in their general appearance.