RE: Indecisive flyer needs advice from the speed freaks...
Personally I prefer the Lanier Preditor over the Viper (flying wise), but they both fly well.
Use the Magnum .46 on the Viper.
(the 52 will work fine too - problem with midrange is due to people over-propping the engine - its a speed engine, not a 3D engine)
get a backplate mount from lcsperformance.com - square one. (although I think the Viper comes with one.. or at least it use to)
Prop you want for the .46 is either a standard APC 9x7, 9x8 or an APC 8.75x8W, 8.8x8.5. You want the engine running near 16K rpm on the ground. Same props are good for the .52 as well.