In peer-to-peer comparisons, the Lancer Evo has always been ahead of the Impreza Turbo. The EvoVII was so far ahead of the "Bugeye" Scooby it wasn't even funny. The EvoVIII is a better car than the EvoVII, but the Impreza III (looks like a Hyundai Accent, but still better than the bugeye) is so vastly improved over the Bugeye that it makes the gap as close as it's been in recent years.
The "one-off" models are more interesting. The Catalunya whipped the EvoIII, the P1 eats EvoVs for breakfast and the 22B is what all the FQ300s have been trying to be, but failing.
My brother drives the Impreza consistently rated as the best driver's Impreza (up to the death of the Bugeye) available in the UK - the RB5 (442 examples made) - but he pulls out of the way of it's peer, the EvoVI.
The EvoII is possibly my favourite of all of them - for various reasons. The Impreza Spec C is just a different gravy, but unavailable in the UK. It would certainly whip the ass of any Evo, bar Mine's versions (and other vastly tuned cars). Anyone who tunes one themselves is a bell-end though.
But this is turning into a "Cars" debate and not a "GT3" debate. In GT3, I'd go for any Impreza, any time - I just like the look and handling more.