Definitely Scar-Veteran on Carnosaur is the best choice for the first Start Collecting box, especially if you plan to use Malign Socrery artefacts.
Troglodon is a specific monster. He is rather cheap, can shoot, has bonuses to charge if shooting was successful, debuffs morale of nearby enemies and can unbind. He does a bit of everything but nothing as good, as other monsters. Moreover, he is not a character and can use command abilities. He needs some more experience with a game to use effecitvely and even then stays a fun choice.
Carnosaur, on the other hand, is a pure murder machine, especially with its jaws. Why ScarVet over Old Blood? Scarvet is a bit cheaper and a core of Firelance Starhost battalion which is easily collected with 2 SC boxes (15 knihgts + Scar-Veteran), which can be developed to strong Dracothion's Tail meta-battalion. His command ability is benifical for any saurus unit, especially big one and he has more protection with his shield. Old Blood, while has shooting attack and some more stats, will buff with his CA ability only himself and other Saurus Characters so needs a heavy saurus army to fully utilize his potential. He is also part of larger battalions: Bloodclaw Starhost and Fangs of Sotek, which will take some time to gather effectively, unlike easy Firelance. He also doesn't ignore Rend -1 like Scar Veteran does, while has the same wounds and save characteristics.
IMO, best choice is Scar-Veteran with Warblade. Give him one of the -3 Rend artefacts from Malign sorcery and you'll get 3+ to-hit, 3+ to-wound, rend -3 monster.