I don't know that Sonny is actually better for Carly than Jax is, but
Carly knows that Sonny will love/care about her no matter what she does!
With Jax I think that Carly feels like it is only a matter of time until she says or does something that will send him running!
How can that be good for you? Always waiting for the shoe to drop?
And now with Jax being so totally unconcerned about the kiss, I'm just about convinced that Jax is holding on to Carly just to prove something to Sonny!