First of all, we must compare the Scar-vet and the Oldblood.
leaving aside that, unless you're playing with WYSIWYG, you can use your carnosaur as Oldblood or ScarVet, the two got different profiles.
The Oldblood does more damage, even its carnosaur delivers more damage than the scarvet's (at the cost of a weaker defense).
The command ability of the Oldblood buffs heroes, including himself, so basically an Oldblood on Carnosaur is far more deadliest then the ScarVet.
The ScarVet, on the other side, got a command ability that buffs other units, so it gives more goodies to your troops.
They have two different approaches and different utilities, and you should use the one that serves better your army.
The Sunblood is a great General, with a powerful command ability that buffs troops and also single saurus models.
Unless you're using a battalion that lets you employ command abilities of different models, I'd say that a Sunblood with a Scarvet on Carnosaur is not exactly an optimal choice, while a Sunblood with an Oldblood on Carnosaur works better (keeping the Sunblood as general)