Funny thing, my very first recumbent (DeFelise Bitubular) in 1990 was an under seat steer set up very much like the LongBikes Slipstream that you recommend and I found it very relaxing. But I find that on a long ride, I want quick access to maps, GPS, telephone and camera, something that a above seat steer bike allows with a handlebar bag/fairing. I want to be able ride along and consult the map, and not have to stop and dig around in the bag. Sometimes in towns, you have to check the map multiple times to stay on the route, and to stop each time is a pain... As for the trailer, as I stated, I already have a Quik-Pak and love it. I bought it for use with my Catrike and have not tried it with my current Bacchetta Strada, but feel assured it would work find with it as well.
Tractor Tom in Okeechobee, FL