Either one is a good call; stick with Associated and get the LRP/Reedy or go with Castle Creations and get the Mamba. I love my Mamba system, but I also know a lot of guys who swear by Associated's LRP/Reedy. Either way's a good call, so pick whichever you think is the best deal.
To my knowledge, though, the LRP/Reedy cannot be programmed as much as the Mamba25 can. Maybe later on down the road you'll want that capability? Just keep that in mind.
Cheers,
Kyle.
edit: and just like in xmods, it's a motor, not an engine.