Pridemage vs Sparkmage + Collar

[Carnac]

I have Baneslayer and Qasali Pridemage in play.

My opponent has Cunning Sparkmage.

During his turn, he plays Basilisk Collar.

In response I destroy the collar by sacrificing Pridemage.

He then respond attaching collar to Sparkmage and shoots my Baneslayer.

I tell him he cant do that because attaching happens at sorcery speed, while Sac´ing Pridemage happens at instant speed, so the Collar will be destroyed before he can use it.

He says he can do so because he has priority.

So who is right? Me or him?

[BC]

Mostly you're pretty much right.  He plays Basilisk Collar, and it resolves, then he has priority again.  You would not be able to activate Qasali Pridemage at this point until he passes priority to you again.  Assuming the next thing he does is equip Basilisk Collar, you would then get priority before the equip resolves.  At that point you could activate Qasali Pridemage to kill the Collar before it is on the Sparkmage.