[Otto]

While I understand that the SMS-1 works nicely, I might suggest getting the BFD 1124. It's very powerful, works seamlessly with REW and has a loyal following here.

For external soundcard, I believe the SB products work well. Yes, to use the ECM8000, you will need a preamp. You should get similar results as long as you use the appropriate correction file for the "mic" you are using -- be it the ECM8000 or the RS meter. The ECM8000 is flatter, and I would expect it to be more accurate overall (never used one), but the RS meter will work OK. If you use the RS meter, you only need the sound card (no preamp needed).

If you want to tweak, I'd suggest the BFD. I think of the SMS-1 as a do-it-for-you device. I know you can tweak it as well, but that's just how it is in my head. One benefit to the SMS-1 is that it has a remote control.