I'd say the constallation also plays a part in this choice. Especially in Koatls claw it is important to choose the right General who can refund CP he uses on saurus units on a 4+.
Koatls Claw gets a +1 innately, as if they do not charge, a CP can be spend for +1 to hit. This means all heroes in Koatls claw got +1 to hit, while the old bloods also have a +1 to hit command. This can be good, but will also be sort of a waste for some warscrolls to stack up on, instead of a Scar vet providing exploding hits, so you can use the same general to use both commands and possibly refund them.
Also the Svar Vet weapons provides some synergies with the available artifacts. Koatls claw forces the artifact giving 2 mortal wounds in addition with 6s to hit, that is pretty good on the spear with 6 attacks.
They are equipped in a way having 2 is not a bad idea, even in Koatls claw if you make the scar vet your general and give the artifact, another carno with an old blood is not a bad choice, as it is a bit more fighty without any artifacts needed. Due to the sunstone spear the oldblood is also a slightly better option for the great drake +1 attack.
That said the old blood on carno is not part of any battalion either, while the scar veteran in a firelance can be on a carno, especially in Koatls Claw getting +3 to charge ranges is a big deal to ensure that +1 hit on charges.
The 40 points is a big deal though, and it might in the end come down to what you could get for that in your preferred list.