I havent used the PI reg but evolve have never failed to impress. ive had a couple of there bolts and there always very helpful.
As for the AKA reg i think there proberbly overrated, i remeber seeing a graph with the recharge rates of a lot of regs, compaired to the AKA 2 liter. from what i remeber they did't even have a CP reg on the graph, which is suspicious as its possibly the most popular reg about.
in the end i think it comes down what motivates you more price or looks, evolve is cheaper than the AKA by the time you get it to your door. AKA looks a little better to me though.
Personally i would buy a modern second hand reg because you wont experience shootdown. im surprised you get shootdown off the ION reg as ive never serviced mine and its still gives very good results it may be worth giving it a good service (new Orings) as you will not get much of a return on ION upgrades when it comes to resale.