I have an S300 on one of my bikes. It's a good crank, worked well for me from day one. I don't have thousands of miles on it, but I have at least a few hundred. Whether it's worth $135 is a question you'll have to answer for yourself. I'd certainly pay that much for another one before I'd pay $220 for an Omnium.
Of course, both models (and others) are hollow-spindle, external bearing designs. If you like that setup, the S300 is nice. The actual spec for this crank/bottom bracket is SRAM's Giga-X Pipe, aka GXP. As Seau said above, there have been reports of problems with the BB that comes with it. I've not had any trouble with mine--yet.
If you prefer square taper spindles, the S300 is obviously a hard pass. Another consideration is the chain ring. S300 uses 130mm BCD, and comes with a narrow (3/32") ring. Is this the kind of setup you want? If so, I think you'll be happy with it.