According to my experience, RPM blast is slightly more powerful. I have played with both and prefer the PHT b/c I am also a hard hitter. I find that control was slightly better with PHT than RPM. TW did rate PHT power slightly lower than RPM, but PHT was softer in stiffness.
Both strings are quite similar though with PHT a little less expensive. I have came across other findings where results in contrary to mine and TW's. I guess other factors are involved... tension and racquet stiffness as well.
I did find a great alternative... Technifibre Black Code and Pro's Pro Black Out (way less expensive version of Black Code). They play almost identical (only diff is the name). You can just pound balls as hard as you want and still have great control and spin production. The reason why I've been sticking with these strings is b/c they are much easier on the arm than PHT or RPM, and durability is slightly better.
Pro Staff RF97 (2) / Prince Response 97 (2): T1 Firewire, Tour Bite, Tour Status, & VS Gut x/ Big Hitter Blue