More or less. The Power is a touch stiffer and more powerful version, but they say it's more durable. The regular Xcel is a bit softer and more rounded. The Xcel Power should therefore give you a bit more spin.
Poly string is a different type of material altogether whose goals are completely opposite of yours: stiff, very low power, very high spin, short playability span. They're made for big hitters who can generate lots of racquet head speed and therefore get lots of spin for their dollar. However, if you'r enot a string breaker, poly really has no advantage. People say "but I don't break strings and play better with poly, so that doesn't apply". Not so. People break strings because they a. hit very hard, b. hit with a lot of spin, or c. both. If you're not doing any of the above 3 to the point that you break strings continuously, then don't bother with poly. This is completely different than a high level player using synthetics by choice, however.