I should clarify it's the BB synthetic gut and yes I can get that cheaper by the pack than with any Signum by the pack. (I have a corporate account with BB).
To be honest I couldn't really tell a huge difference between either signum as a cross. But I haven't got the same experience of playing with them as with all gut and gut/synth gut hybrids. They felt very similar and both have less power than the monofilament synth gut/gut hybrid (which again has less power than a full gut string job). I know this because the first time I played using the signum/gut hybrid was in a match and I dumped a whole lot of touch volleys into the net in the first two sets before making adjustments for the last set. This was about the only place I really felt the difference - serves didn't seem to be any worse off (and it may have been my touch was off that day though I do play with a lot of touch shots).
As for feel, I think that monofilament synth gut/gut hybrid has better feel. I get the impression of how the ball brushes in the strings better than with the signum/gut hybrid. Comfort was very similar though it's not something I routinely think about except when I notice my arm/shoulder hurting and I didn't note this at all with either the poly pro/megaforce.
Looking at the strings now (megaforce/gut hybrid) shows some fraying of the gut but no more than I'd expect after two weeks with a synth gut/gut hybrid. Funnily enough as I mentioned it was the pro plasma that snapped first in that particular combination with the gut. To me it just underlines how much more the mains affect the strong job than the crosses.