Uhm, my years of Siege experience taught me that Wendla and scout hero set are best for defending scout waves during SH Siege. I block all waves with one single town using 500% boosts.
Raine and Ubba hero set for the other waves.
Yes this would be my choice if I expected to majorly join in for other than spy stronghold defenses/attacks ... but I intend to be only/mainly doing the spy stronghold defenses/attacks. So I am thinking my ubber would be best for this and I do not plan to do spy attacks for towns.