Frame choice depends on the kite and what you are trying to accomplish.
For the Enigma, the strength, constant bend radius, and stiffness of the PX tubes worked well. Overall balance was achieved with the longer keel and tail weights.
I've seen about every variation used as a spine: straight Pxxx, pultruded, tapered (in both directions), light spine, heavy spine, ....
Testing is important. You may well find several frame combos that work ok and a few or just one that is best.
Another example. As the Enigma leading edge has more curve at and below the lower spreaders, I opted NOT to use a heavier LLE than ULE due to bend characteristics. Both the P1X and 3PT worked ok in the UL. The Nitros were preferred over the P2X in the Std.
Good question. As with many things in kite design - it depends
May I ask you what drove you to use PX spars for the spine instead of the tapered PT spars ? I would think that tapered spars help to move the weight distribution toward the tail where we usualy put weight, but maybe did you rather get a very stiff spar ?
I'm asking because I want to make a SUL/UL Sixth Sense and I'm debating what to use for the spine, I plan on using 2PT for upper leading edge and 3PT on lower leading edge and lower spreaders.