Vehicles seem less vunerable to firebombs - they seem to require direct hits to do anything more than light damage.
It is definetly worth taking the Shiva over the Sulla if you can afford it - the armor protection is far heavier and it has the firepower to seriously hurt ground targets without needing to run for ammo every time it fires.
The RAC/5s on the Shiva A are particularly effective at killing enemy aircraft too.
Note the armor is still fairly light compared to ground units, so head on dives into enemy fire still result in a dead aero, as all the damage is being done to one location.