I use REW extensively, have a
calibrated Behringer microphone, M-Audio micpre, boom mic stand etc...so measurements are not an issue. REW is nice since you it shows you effects in the time domain, with ringing being a problem when configuring the low end response in any room. I like the fact that REW has the resolution even though some would argue that
1/3 rd octave smoothing is not a big deal at frequencies below the Schroeder. I don't think so. If I need to kill a peak, I want to know
exactly at what frequency which is what the Behringer allows you to do. And you can set about 10-20 frequencies quite easily (although if you are, you are probably doing something wrong!). The Behringer also has multiple crossover slope options which also helps (Bessel, Linkwitz, etc.. and anything from 6dB to 48dB/octave, similar to the SMS-1).
That smoothing thing on the SMS-1 really irritates me, but that's probably a personal thing!
Be that as it may, the SMS-1 is cheaper (as you don't need to purchase the mic, etc..., but if precision is what you are after and the usability on MULTIPLE subwoofers distributed throughout the room then you need multiple SMS-1's which adds up).
I think your last comment is the most important:
"The ability to set the subsonic filter in software at home subwoofer ranges is critical too with resonant builds."
And for that reason, I will go for sealed Maelstrom 18's or 21's. A 6-8 cubic foot box for the Maelstrom 18 or a 10 cubic foot box for the 21's with a tiny bit of boost on the low end should more than sufficient and foolproof. A Face Audio amp and that subwoofer system will be a home run easily.
It's all about how much or how little work you want to do, what you are comfortable with and if you have time.
Anand.