From a collectors standpoint first and foremost, terradons. I know they are pricey per model,
but even in the second hand market they are crazy expensive simply because no one sells them
unpainted! That tells me GW is probably the only major market that still has them available.
From a gamers point of view, for a lower point amount, especially, you want to go with
chameleon skinks, their level of harrassment is unbeatable, and talk about a redirecting unit!
As points go up from say 1k, then terradons begin to really fit the bill. Their flightiness really
adds a whole new tangent for the opponent, especially if he already is contending with either
a steg or a slaan (or even a carnosaur) all on top of a full unit of chameleon skinks raining
poison darts down on your opponents head!
So you will probably want to have both, so just go ahead and get the terradons as you can, and either
green stuff the chems, or hit the secondary market for a unit of them, couldn't run but maybe $15 USD
or #10 GBP; well that is what I paid!
GL, Happy Gaming!