I've built them both.
Wayne Kellner deserves an award for his design of the Orbital Transport. It's a bear to build because of the sheer amount of lumber involved, and said lumber can make finishing tricky, but if you take your time with the build it is very rewarding and guaranteed for some applause at launch events. My glider has a tendency to simply hang in the air...it's pretty cool!
The Semroc Space Shuttle isn't as hard to build....the hardest part for me was getting all the dowel/launch lug interlocking portion done right.
Flights are interesting....mine is prone to swoop and swirl a bit during powered ascent (I've had better luck with a B6-2 as opposed to a C6-3), but the glides are a different story. The upper glider flies beautifully, which is strange because looking at it, you'd think it would drop like a rock. The larger ship glides well IF you have adjusted the elevons correctly just prior to launch.
Both are excellent kits. Overall, I'd give the nod to the Orbital Transport, but that's only because it has been a favorite of mine since the 70's.
I'd love to see Semroc provide an upscale version of the Space Shuttle....that would be interesting..