Personally speaking, the storage compartments on stocks like the EMOD and Modstock from Vltor and LMT SOPMOD is less important than the cheekweld I get from the stock; the balance it provides to the overall weapon, and "rattle" factor (the stock wobble when locked at any of the four-to-six positions on the buffer tube).
That said, I have used Modstocks, Magpul CTRs, and SOPMODs on my carbines and they are all good-to-go. The CTR is one of my favorites since its cheaper than the SOPMOD or Modstock (at least when and where I bought it) and it locks up tight with no rattle. Magpul's UBR is supposed to be a slick stock and may get one when I get a mid-length upper. Magpul now has the "MOE" stock which is a "CTR Lite" without the locking lever and QD swivel attachment point; both of which are important to some people; not so much for others.
The EMODs supposed to be really good too.
If you don't know this already, but the stock you get needs to conform to the buffer tube; either commercial or military.
BTW, I don't consider the storage on the stocks critical because I'll either have spare batteries, bolt, and spare parts on my vest (with mags) or a "go" bag. However....your mileage may vary.