Lots of things to consider.
1. you can only get the mirrored version of Black Flame if you are in the delivery zone of XFS (the Indiana area) or if you're going to buy the paint from XFS and get a mirror and spray it yourself. So the mirror might not be an option (my situation as well.)
2. Black Flame is a gray/silver screen for better black levels and better ambient light performance. High Power is just that HIGH POWER. It has much more gain than the XFS black flame (at least the PVC version, I'm not sure how much of a boost you'd get out of the mirrored version.)
High power is very reflective and pretty amazing. It does require that you shelf mount your projector to get the gain boost and it does introduce a viewing cone. The viewing cone is no big deal. Still plenty of gain a bit off axis and way off axis the screen is as bright as a normal white screen. The biggest thing to worry about is the gain in terms of black levels. A projector without an outstanding "real" contrast ratio is going to be leaking too much light to maintain good blacks during dark movie material. My HD1000U was not very effective with my test piece of High Power. You can see a picture here:
The trick with black levels (absolute black levels) is that normal video material that has a lot of brightness will still show good blacks. This is deceptive. Movies are a better test for black levels than HDTV. If you're watching mostly sports I don't think you're going to see as much of a problem with black levels. But if you watch Underworld it can be. Without lots of brightness the true black level ability of your projector will appear. With High Power that appearance then gets amplified.
XFS Black Flame will help your black levels much more and help your ambient light viewing. But if you don't care about black levels and want power, the High Power can also help with ambient light (provided your lighting is to the side of the screen and not in front) by simply being so much brighter (if you're shelf mounting). The choice is yours. Selecting the right screen is about understanding the trade offs each screen makes and fitting the right one into your situation.