I've just finished building the new Serpent Natrix 748 and all I can say is that the kit is the best quality kit I've ever built. I've already driven a buddies car and it is very easy to drive and quicker than my outgoing 747 which was also a fantastic car as well. I had the Mugen MTX-4 and MTX-5 briefly and I wasn't personally impressed with it to be honest.
All this however depends what you can get support for locally. It is a PITA if you break something you don't have at the track and you can't get a spare locally and having to mail order everything!
The Xray is very expensive (certainly locally) and most people who I've seen run it needed to buy this hop up and this upgrade in order to make the car quick. The Serpent falls out the box quick and you don't need anything extra really apart from the usual shock springs and things to setup the car to your specific track conditions.