Which K frame would you use, given the choice?

Unless it's a true ''resto'', and given the choice, I would use the '73 and up spool mount K member every time.
The same goes for the front suspension and BBP brakes.
They are better, stronger, and parts are cheaper and more available than the crummy SBP stuff.

That being said, a lot of people don't know that the real reason Chrysler went to the spool mount K frame and trans mount was for collision purposes.
Primarily, In a front end collision, the engine would stay in place instead of shearing the mounts and shifting forward.
The other benefits were with roll overs and side or rear impacts.
The engine and trans would stay in or close to their original positions.