Haven't tried the omega pedal before but I'm using a Blockhead Rhythm Maker. $30 and a splitter is built into the thing. Passed Painkiller 4 or 5 times with this thing and so far it's been holding up.
Blockhead - Rhythm Maker Bass Pedal with Splitter for PS2, PS3, Wii and Xbox 360 - BH4010
Personally, I wouldn't spend a crazy amount on a pedal for a game. At that point, I'd just buy the damn instrument... which is why I ditched all my plastic guitars.