I feel you should the make third mode "Counter target instant or sorcery spell" instead, since the 'unless its controller pays " looks and feels inelegant as the other modes don't have it.

Doesn't seem like escalation is going to often be useful here - are there often going to be abilities and spells on the stack needing to be countered?

I agree, KeresAcheron, but I wanted this to be something of an answer for Eldrazi cast triggers. This answers Vitenka as well, I suppose.

Might it be simpler to recombine the activated and triggered ability modes? I like the idea, but it seems like an unfair tease to suggest you may get "counter unless " when you'll only ever really be able to use two modes.

But what if your opponent cast Lightning Bolt triggering Guttersnipe, and then tried to copy it with Dual Casting?