I have the NTC3 as well as a Serpent710. Both are great cars but they have their differences. The NTC3 is shaft driven as opposed to the 710's belt system.
The NTC3 is a great car. It allows the driver to grow with the car, to a point of course. Tee adjustability of the suspension, transmision and hop ups are great. It can be used as both a parkinglot basher as well as a contender for racing. I have done both.
The Serpent710 is a great car as well. Out of the box, it's performance does surpass the NTC3 as far as agility and handeling are concerned. There are so many adjustments to make on this car to make it perform according to your driving style as well as track conditions (DRS, roll center, suspension, shock dampening positions). Of course, it will overwhelm you and as with anything else, do one thing at a time. Generally they say that the 710 is made for a more "advanced driver." I think this is true if you are going to race but in reality, this car is for the more patient driver. As I said, there are so many settings to do on this car. Very few hop ups since the car comes with a 4mm chassis and almost everything is either plastic or carbon fiber.
The MTX-3 is going to prove to be the "faster car" out of the box for those of us that like to build then race. I have seen quite a few of them at the track, including Team17's. Great performers. I just can't give a more in-depth description since I don't have one. As far as I can tell, another great car. Just not as many adjustments to overwhelm you with.
If this is going to be your first car AND you plan to just bash it in the parking lot. I would suggest the NTC3. As I said, it will allow you to grow with it. IF you want to get a higher end car, then I would suggest the MTX-3 but be forewarned, parts are pretty hard to find at your LHS. On-line shopping will be your best friend. The Serpent710 is a great car as well but can be a handfull trying to master, but just as the MTX-3, these 2 cars are definite performers.
As far as the HPI on-roads, I don't own one so I can't give any accurate information.