Both serve quite the same role in the army but do that in a very different way. I find chameleons to have a bit more staying potential but terradons have better movement and are a bit better in close combat.
Now that you have 2x12 skirmish skinks maybe you could reduce your unit sizes to 10 and then convert the spare 4 to chameleons or maybe simply by painting them differently. You would just need one more somewhere and then get the models for terradons. I use normal skinks as chameleons. I have just paint them to stand out so they won't be mixed with normal skinks.
I never drop the rocks to bigger units these days. I try to find some more suitable target for them. Maybe some small skirmish units or fast cavalry. Units that have low or no saves are the best. Sometimes I even drop rocks warmachines too and hope for 6's.

But this is only if I can't charge because of intervening unit or something.
BR
Agrem