Since we had a discussion on mounts a few days ago at my local gamestore, I did some numbercrunching here
Carnosaur vs Sun Dragon
Let's assume swiftstride, because that makes them equal in points.
Carnosaur have more movement, but isn't a flier, so the sun dragon wins here. It does make it suscepticle to heavens spells, which is worth a mention. swiftstride helps alleviate this a bit.
Sun Dragon have +2 WS, which will almost never make attackers hit at 5+ outside of fear, so WS3 and WS4, it'll have the advantage, and then again vs WS7.
Carnosaur have +2S. That's a massive difference.
Sun Dragon have +2I, which is a huge benefit against other monsters.
T, W and A are the same
Now, special rules. Both have terror. Sun Dragon have 1 point better Scaly skin save. Now, I said attacks are the same, but they are not. If the Carnosaur does even 1 unsaved wound, it gains frenzy, and cannot lose it again. Being coldblooded and Ld8 (because of the oldblood), this is rarely an issue from my experience.
What it comes down to, at least for these two mounts, is the diverging special rules. Aside from fly, the sun dragon has a breath attack, which makes it fairly good against infantry and hordes.
The Carnosaur, with it's +2S, also has D3 wounds, which makes it BRUTAL against multi-wound models or monsters. Ogres direct EVERYTHING at killing this monster before it hits cc, because if it does, everything they have start dying horribly.
I know this particular monster cost less than the carno, but my recent battlereport was me against 2 elves on griffons. the griffons are actually surprisingly close in cost to a naked carnosaur, if it takes all upgrades. Since these are +1s on the charge and ASF, they pretty much always have these upgrades, which puts them at 25 points below the carno. Even when charged, I shredded them. They don't have the attacks to deal with another monster, much less a character like the oldblood, which means the carno will almost invariably counterattack, which hits on 4s, on wounds on 2s, multiplying into d3 wounds. Without frenzy, that's an average of 1,6 wounds, before multiwounding. With frenzy, you instead get 2 wounds, which hon average becomes 4 unsaved wounds. That's the best possible outcome a griffon, or even a sun dragon, can hope for, because that's their amount of attacks. the best outcome for a Carnosaur is 15 wounds, so even considering "best case scenarios" here, the Carno still pulls its weight.
The carno isn't exactly a godlike monster. It's not a star dragon, that's for sure. But the fact that it can actually do pose a threat to it, despite costing almost half of it, is pretty amazing in my opinion.
The only bad thing about the Carno, or any other monster, star dragon included, is that Cannons can reliably one-turn-kill them. But that's the way the game currently is, unfortunately.