The Longbow Heavy in RV does significant damage against the Pillboxes (and the stray villain who tried to get me while I'm working on the pillboxes). Not sure of the exact numbers though.
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Arcs on Live
#1460 Hometown Rivalry
HVAS is a monster of a tank who can be buffed and healed, wish you could get a permanent one as a buddy for defenders like me who can't solo 
I only used longbow in the RV but it seemed weaker than HVAS, then again I like HVAS because it looks like a giant armored purple Dog from Half-Life
I'm pretty sure they have different powers, though they would share a few 'giant mech' standard attacks.
The Longbow and Arachnos Heavies probably have more in common.
the longbow pet in question is NOT the pet in RV that you get while in zone, he's referring to the one you get as a temp power available outside the zone for getting 1000 points before the zone goes into reset mode.
as far as I know they're identical except the Vanguard HVAS has a 50% boost to damage, and a 100% run speed bonus through it's Heavy Boost ability.
I've looked at their listed powers on Paragonwiki, and they are identical. Are the pets themselves indentical, save for appearance? Also, about how much damage do they actually do? I've never had either, personally, but I've heard stories that they put out very impressive damage (Two of them quickly dropping archvillains with a little help, among other things).