I've only tried Scorpion in full bed and as main to gut x. I've not tried SPPP, only their tornado and hyperion in full beds.
I would extrapolate those experiences to guess that Scorpion will be the stiffest of the three, great for directional control, and SPPP would hold tension best of all as a poly cross, albeit less powered than MB.
I have noticed thinner crosses do well to enhance playability in a stringbed, and is why I always come back to 1.20 and thinner polys to accompany my guts.
I'm not sure if I answered that well though, since I've not used those strings as crosses...
Head Pro Tour 280/630, MP 18x20. Tour Bite 18/Mosquito Bite.
Sampras serve, Agassi backhand and Federer forehand. heh heh