Full disclosure: the troglodon does not impress me. I mean, the model is gorgeous, but I find the rules underwhelming. Nevertheless, let's do a side-by-side comparison and we'll see if I can sway you my way of thinking.
Ranged Combat
Troglodon
• Spit Poison: 18'' range, Strength 5, deals d3 wounds if it hits, Quick to Fire means that it's a good mobile firing platform and a bit scarier to charge... however, that attack is from a BS 3 platform. Poisoned Attacks helps a little, but only a little, because you're still hoping for that magic 6.
• Javelin: 12'' range, Strength 3, Poisoned Attack, Quick to fire. Similar abilities, but only a single shot and a single wound.
• Spells: Potential to arc damaging spells through the Skink oracle.
Ancient Stegadon
• 18'' range, 2d6 shots, Strength 3, Poisoned. This is a lot tastier. The high number of shots - on average, around 14! - mitigates the lousy Strength and BS.
• 1 Javelin... eh... not too impressive.
Overall: The ancient stegadon wins hands down. Like many Lizardmen ranged options, it's cursed with mediocre BS and lousy Strength. However, with its potentially huge number of attacks and Poison, this problem is mitigated somewhat. The troglodon, on the other hand, needs to hit with that one attack in order to enjoy the benefit of Multiple Wounds. Like the giant bow on the baby stegadon, you are going to miss that attack pretty regularly, especially if you want to move and shoot, or shoot at long range... and with 18'' to work with, you will pretty much always be at long range.
Melee Combat
Troglodon
• 3 attacks, Strength 5, at I2, Poisoned, plus a Strength 5 Stomp. Predatory Fighter is nice, but unreliable. WS 3.
• An additional Strength 3 attack at I4 from the rider. WS 2.
Ancient Stegadon
• Between 2 and 7 impact hits which can be upgraded to cause d3 wounds each.
• 3 attacks, Strength 6, at I1, plus a Strength 6 Stomp. WS 3
• 5 more attacks at Strength 4, I4. WS 2
Overall: Woah, does the stegadon ever win again wiith the potential to hit up to 16 times in the first round of combat! Ok, that's not very likely with a mediocre BS, but those Impact Hits are nice, especially with the upgrade, and has the potential to sway the combat in your favor before it's even begun. The Skink crew have enough attacks that even with their bad BS and poor Strength, there's a chance of them contributing. The troglodon, on the other hand, suffers from a similar problem in melee that it does at range - just not enough juice. The Poison is nice, but Poison thrives in situations where you have many low quality attacks. 3 is not many. By contrast, the stegadon gives you many high quality attacks. While many low quality attacks + Poison can be > few high quality attacks. Many high quality attacks > few high quality attacks + Poison.
Durability
Troglodon
• 5 Wounds, T5, 4+ Armor
Ancient Stegadon
• 5 Wounds, T6, 3+ Armor
Overall: Do the math. Stegadon wins.
Utility
Troglodon
• Can arc spells. Not bad on a M7 platform.
• Primeval Roar. Makes getting extra attacks from Predatory Fighter more likely. Not bad, if you can position it well.
• Diving Rod upgrade. An extra chance to get a power die... but still just on a 6. Not reliable enough to be worth it.
Ancient Stegadon
• No particular utility, except for having an enormous base.
Overall: Troglodon wins this category.
• • •
My opinion on the troglodon is that it tries to do too many things and doesn't do any of them well enough. It's got some neat utility bits, but isn't quite as useful as, say, a priest (or as a mount for a priest... which would have been nice). It's not a close combat monster like a stegadon. It's got a ranged attack that's trying to have penetrating power, but the low BS and single shot makes it too unreliable. If it did any one of those things very well - say it lost the ranged attack but had two more close combat attacks, or had a lower Strength but a better range and BS on its poison spit, or could serve as the mount for an actual Skink Priest rather than a moderately interesting pseudo-priest - then it would be a stronger combat, ranged, or utility piece.
As it is, it just... flops. Jack of too many trades, master of nothing.