X-One Bi-Phase is the best (by far) of the Technifibre multis with which I have hit. I was impressed with the crisp feel, responsiveness and it holds tension better than any of its Technifiber siblings. I would rate it in the top 5 multis currently available, perhaps even in the top 3. It is an incredibly solid string. What it has lacked for me is the "wow" factor. While nearly all who have tried it have had praise for the string, it just isn't one of those strings that people remember and have to have. They like it, but the price is a little high.
That said, the Pro Stock is similar. It is even more expensive than the Bi-Phase, perhaps not quite as durable, but is specifically designed to play more like gut. The micro-tissue technology costs more and while it makes a great substitute for natural gut in weather/court conditions that might harm natural gut, it does not quite reach the same performance as gut. It plays a little livlier than the bi-phase and caresses the ball a bit more, but again the price makes it a tough sell. If this string were in the $10.00 - $13.50 range it would be wildly popular. Bottom line is it is expensive and doesn't play as well as a natural gut hybrid, IMO, but does comes closer to natural gut than the multis that are currently in wide spread distribution. Both are fine multis and you won't go wrong giving them a playtest for yourself to see which you prefer in your racquet.