An external dump on an internally gated turbo is useless and silly, if you ask me. For one, you're going to get that loud ass external sound even at part-throttle boost levels since internals open slowly a little at a time instead of a really quick opening like an externally gated turbo. So an external dump on an externally gated turbo is fine in my book, but not with an internally gated turbo.
Go with the 3" turbo-back for $180 and don't get the o2 dump.