Probably depends on your list I guess. The main differences is that the Oldblood gets a ranged attack, a different command ability, a different weapon, and the Carno's bite does more damage, however he loses his shield. The ranged attack is nice, but it looks inconsistent since it's D6 attacks. If you have a lot of Saurus heroes the Oldblood is probably better, but most of my heroes are Skinks or Slann so I built the Scar-Vet. Also because it's cheaper and fits into my list a lot easier.Looking at the models on the sprue close up and I almost regret my decision because the Oldblood parts look cool as fuck. That metal arm, that spear.
My real question is what weapon I should equip my Scar-Vet with. I chose the spear because of the rend, but does a 2" range even matter for a hero, since he'll probably be in base contact anyway?