It is my opinion that when the RS gold update to the cb begins, summoning will not be as effective as herblore, however after the revamp of summoning it should become a lot better.
Disregarding everything above I'd suggest using the xp earned on summoning because summoning is more benefit from the bonus xp because you can save money as well as time from collecting charms. Do you remember how summoning had the 1.1x multip