Honestly, either one is fine and a worthwhile unit to have. Which one you need depends on your army and your playstyle.
The Oldblood gives a range attack and a little more damage per attack. His command ability is focused on Saurus heroes, so he works best as a general for an army with several Saurus heroes. The downside is, his defense is weaker due to his lack of stardrake shield.
The Scarvet, on the other hand, has the better defense, but loses the range attack and some damage output. His command ability buffs a single unit of Saurus.
It depends on your playstyle. For me, I prefer the Scarvet, but could see the Oldblood as a glass cannon and/or used to sit back and buff the other heroes in a bloodclaw starhost.