I was faced with this exact same decision a month or two ago and I had the financial resources to go in either direction. Finally, after reading as many posts/reviews as I could find as well as chatting with current owners, I elected to purchase the SR-404/SR-313 combination (from Koji at EIFL). My biggest worry had been the bass response which many writers suggested was a weakness in electrostatics and especially the SR-404. My experience, as well as many other SR-404 owners in this forum, does NOT corroborate what a few have reported (the rumor just won't die). I find the bass to be rich, deep, tight, extended, full, lush, and even impactful. As a drummer, I am extra sensitive to the punch of the kick drum and I get plenty of impact from these SR-404's, even more so than my HD-650's.
Since purchasing this STAX system, I have had ZERO regrets about the Omega II; in fact, I am no longer craving any other headphone. I believe that I have now attained a kind of state-of-the-art aural experience with this STAX rig and there is simply no sonic feature or quality that I feel is missing or lacking. Now that I understand how the SR-404 performs in the bass department, I wouldn't want more, which is the most often quoted strength of the Omega II vis-a-vis its siblings. Similarly, the treble extension on the SR-404 is wonderful and I wouldn't want less, which the Omega II's are supposed to offer.
So, for me, the STAX SR-404 delivers every sonic characteristic I want in a headphone. My only solitary gripe is the itchy/scratchy foam liner inside the earphone housing. After 30 minutes or so, I literally have to remove the headphones and rub my ears! One of these days (like this weekend maybe), I'm going to pull the pleather rests off and replace those brillo pads with something soft and luxurious (satin?). I've just been reluctant to mess with near perfection. I would hate to ruin the sonic qualities by changing that inner lining. If I knew what function it performed (e.g. dust/hair trap?), it would help suggest alternatives that are easier on the skin than burlap or whatever that coarse material is...
KenB