For what it's worth, I do own both and have them in front of me at the moment. I've had ample opportunity to compare the two with both tube and solid state amps. The HD650 and the SA5K are both excellent, IMO, but as others have said, they are very different animals.
The Sonys are bright, forward and colder. The Senns are darker, detached and warm. Sonys are fast and crisp; the Senns are less so. The Sony bass response is sharp and transient, very contained, very on-off. The Senn bass seems to appear earlier and to go deeper with more authority.
I prefer the Sonys for most of my listening, but I'd recommend the thread starter lean toward the Senns. He likes the Senn house sound, and it synergizes well with his present setup GS makes a good point about the tube amp. I think the SA5K works better paired with solid state equipment that reinforces the speed and brilliance of the sound. The Senns have an affinity for the warmth of tubes.
As far as build quality is concerned, I've seen no evidence that the Sony's are deficient. There was one report of breakage that I recall. They probably aren't as sturdy as the Senns, but I wouldn't expect them to fail in normal usage.
Obviously, auditioning both headphones would be optimal, but the Sonys seem to be a bigger risk than the Senns if they must be bought without being heard first.
All IMO, of course.
Regards,
BW