I have a Serpent so I may be biased. However if I had to do it all over again, I'd still choose Serpent. I'm not loyal to Serpent, like all racers on a budget, my loyalty lies with my pocket book.
: A good car if you have the current latest version. Parts support is somewhat scarse but you can always find it online. Car is durable but like all things not indistructable nor prone to wear. Parts are expensive.
You will have to reach deep in your pockets to fund your spare parts and options. I know this all too well because I run an XRay electric TC and run into this problem all the time when I need parts.
: Its been a good car for about the last 4 years or so and people are still winning races with it. Parts support is good depending where you are at and parts cost is very economical. Sounds like a winner eh? Not really. The MTX4 and all its variants are at the end of its product life cycle. The MTX5 is just around the corner. Although, people will say that there will still be parts support for the MTX4, don't short change yourself. I was told the same about the MTX3 up until last summer where I desperately tried hard just to find gears. Once the MTX5 comes out the same will happen to the MTX4.
: This car is a new design that is not a "me too" like the above two cars. Although there were some parts issues after its introduction, the problems have all been solved and the kits are now updated with the latest improvements in parts. This can be said of any new RC car where there are always a few bugs to work out. Nevertheless, the car is easy to work on and responds well to tuning changes. Parts support is also good depending on where you are at and parts prices are about as economical as Mugen.
So there you have it. I'm sure, if there is still parts support for you R40, you'd still hang on to it. However, all things must come and go. Just like the MTX3 I had before the 733.