i have a SBR Oly upper. sten mags are a 5-1 price point over either RRA modified uzi or Colt specific mags. The sten adapter is 1/2-3/4 the cost of the many different mags for the colt style. Sten mags function 95-100% of the time vs. the "many tweaks" possibly needed to get the colt/rra to run 100%.
you will see some "issue" related threads here regarding the colt/rra system. Over on ar15.com in their pistol caliber forums you'd spend a few days (ok many hours) reading all the "how do i get this to run right" post regarding the colt/rra's.
the colt system will cost more up front then the oly . your choice.