If you haven't already bought it get the Saito 82. Low weight and great power for the Reactor.
Otherwise the 91 setup is about 7oz's heavier and you'll have to move your battery further back to balance. Tank in the belly... it'll fly. She won't float as nice but it'll fly. The additional power will let you hover with a little more authority.

The 55AX would be faster but less thrust, the OS91FS would be more thrust though heavier where as the 82 is right in between both with less weight.