You mentioned about the Electra vs. the Ion....there's no comparison. the ion can way outperform the electra any day. The rocking trigger frame is overrated, and is very unreliable. Now, for the Ion vs. Promaster...I perfer the Ion over the Promaster simply because i have experience with the Ion. The Promaster is basicly the same gun except the body style. The Ion, on the other hand, will not come with the q-lock feedneck, and the HPA line, which is about a $80 value. If you want to use Co2 with the marker you will have to buy a remote line...you need this because liquid Co2 will run down and freeze the electronics. With the Ion, the QEV isn't a necessity, just an upgrade...and also the stock trigger is pretty damn good for a stock trigger.
I'd definatly go with the Smart Parts Ion.