Mmm ... dare I say it? Well, I have both cars so why not so here goes:
Go check out the pictures here
and see how many 710s as compared to MTX3s are being stripped and/or repaired.
Don't get me wrong, I'm not saying one is better than the other. I see this happen at a lot of races and I race a lot. In fact I'm probably too old to win the world Championships by now.
What I do agree with all of the above discussions is that if you're relatively new or a mid-range driver, get the MTX3 because to some extent it is easier to drive out of the box.
However, as soon as you progressed a little bit, you'll see the need to pay for a bunch of hop-ups and do regular maintenance on the car. You see Surikarn's car in these pictures? Its the Pro-Spec version which has all of the hop-ups. Mugen has accepted that this is what the MTX3 needs to be top-level competitive.
Conclusion? Start with the MTX3 then decide afterwards how or with which car you're going to drain out that extra bit of performance from (for your given skill of driving and tuning). I'd rather go with the 710: less money on hop-ups, more tuning options and less/easier maintenance.
And as for Surikarn, he's just a superb driver so he apparently won easily. He probably does well with any car anyway: nitro, electric, 1:10th or 1:8th! Don't forget, he has a FULL-TIME mechanic. All you see are pictures of him sitting around signing autographs. Hey' if I have a full-time mechanic I'd be rather relaxed too and will probably do better than normal in my race right?
My 2 cents' worth!