The promaster operation is just like Timmies, Pimps, etc... HP reg feeds LPR that operates a ram that opens a valve and lets air shoot the ball.
The Omen is a different bird entirely. Look at the stickies for an Omen animation. In the end, though, it also has a valve that directs air to shoot the ball out the barrel.
Unaided by software, 99.9% of players will probably get the same rate of fire out of both of them once they got used to the marker. So the bottom line is that the markers will perform just about identically.
I will say this again: What marker you choose should be more about YOU than the marker. You should choose a gun based on a couple important factors. Number one is probably picking one that you just "like." But you should also make sure that it's going to be one that you can work on cuz if it's not than you aren't going to "like" it at all when something goes awry and you can't tech it. If you are going to pick a marker that you "like" full well knowing that you aren't very good at tech'n or that you aren't willing to take the time to learn, then you better pick a marker that has excellent tech support and customer service so you can get problems taken care of in a hurry - nobody likes to be on the sidelines or shooting a loaner.
So, being that they are both going to offer similar performance and are of similar quality, one is left to decide on looks, and customer service. ICD has good customer service, but it is no where near as fast as Evil's. So, because Evil has better customer service and because they offer color choices, I would pick an Omen.
Don't get me wrong, I think ICD is a great company that offers quality products. They have been in business a long time and in this litigious industry servicing a growing sport that relies heavily on word of mouth, no small company could last if they offered crap products or hosed their customers. My teammate shoots a 'style that just rips. I'm sure that same quality is offered in the Promaster. I just think the Promaster is weak looking - too much like a wannabe Timmy, no asthetic inovation. It's got a lot of bang for the buck. But so does the Omen - more bang in my opinion. So in the end I'd have to pick the Omen even though the Promaster is made in Idaho which IS the best state in the union.