Thanks for the replies @windstruck & @Freda Latou, that's helpful. I had a brief email exchange with Steffen today and his take was:
The NasaStar4 / 4.0m in size has much less traction (without lines) than the StreetStar2. The traction of NS4 increases with long lines.
The StreetStar2 has a ERGO bar, so you can actively work against Backstall, since the angle of attack can be changed with one hand.
For streetkiting on tight spaces, the SS2 is the better choice. If you want a flexibly deployable kite (long and short lines), the NS4 is right.
That seems to confirm what you said about the SS2 having a different profile to the SS1 @windstruck which equates to better traction. Pros both ways so I'm still in a bit of a quandry but there's something about the simplicity of the SS2 that appeals...