In comparing the two Sprinters, Airstream Interstate and the Roadtrek RS Adventurous with the rear sofa design and two passenger seats the plans are almost identical.
Untitled Page Airstream, Inc :: Floorplan Floorplan
From what I can tell Airstream basically cloned this plan from Roadtrek as has Four Winds and Winnebago with their Sprinter B offerings.
You have to look at the specs to make further comparisons. As far as fit and finish goes Roadtrek uses real wood for their cabinets and solid surfacing for their counters vs. Airstreams plastic laminates. Roadtrek has more depth of experience in putting together B vans. Whether that translates in to better design, quality, etc. may be of a minor degree since I haven't heard about complaints from either. It may be subtle, but I've always like the exterior look of the Interstate over most all other B manufacturers.
Did you not mention on RV.net you were considering full-timing? Alone or with partner? That makes a big difference. If full-timing, both the Airstream and Roadtrek sacrifice a lot of space for the extra two passenger seats that would rarely be used. Space is dear in a B. The other things I never liked about either model was the bathroom on the passenger side. When driving I like that side open as much as possible to see cars through the rear view mirror and also to simply look back into the interior. The tall stuff is more suitable directly behind the driver where you can't see anyway. Also, I never liked the refrigerator right on the floor on these designs.
People like their Airstreams and Roadtreks. Don't get me wrong there. Everyone has priorities in desires. With the extra passenger seats they might be more functional for multi-use or use with friends. My preference is the plan (mostly similar between them) offered by Pleasure-Way, Liesure Travel Vans and Great West with the bathroom behind the driver.
Since you like the Roadtrek and Airstream I am assuming you like the plan. If so, then I would recommend you buy the one that gives you the best deal. I think that is what it should come down to because the basic vehicle is identical (MB Sprinter) and the specs and features of almost all the Sprinter Bs are basically the same. Even if there is a quality difference as I mentioned, both will offer good enough service. Bs in general are quality made RVs compared to most other types of RVs. Again, look for the best deal.