Basically, the reason to justify an upgrade would be much faster response time and more exacting precision, primarily. Then you've got better build quality, better range, intuitive menu systems, more features. All secondary, but good reasons too.
The Futaba 4PL can be had for just over $200 right now at AMain. The Airtronics MT4 is around the same price. Both are awesome radios. You won't be disappointed. Top end radios you'll never replace! The Futaba 3PM is a nice mid range radio for $175 or so if you don't want to quite go all the way to the top.
The Futabas or Airtronics (also called Sanwa outside of the USA) would be my first choice. I've got nothing personal against Spektrum, in fact where I race most drivers choose the Spektrums. Not me though, and here's why:
Futaba and Airtronics use a technology called Frequency Hopping Spread Spectrum. This means they will look for an open channel to transmit on, and if it becomes congested or experiences interferences, it will hop to a new open channel. It will keep hopping as needed to stay interference free.
Spektrum uses a different technology called DSSS - or direct sequence spread spectrum. When you turn the radio on, it will select two channels. If the first one experiences interference it will switch to the second. If the second has interference, well you're just plain out of luck.
Radios using FHSS perform better than radios with DSSS. Spektrum is the only manufacturer still using DSSS. Everyone else has moved up to the better technology. In real world use, you'll probably not notice the difference unless you're at a big race with a lot of people, but why start with a disadvantage?
The other reason I don't recommend Spektrum is the brown-out issue. Savox servos and Castle speedos are know to brown out Spektrum receivers. They just don't supply enough power to the electronics, and the system will momentarily turn itself off. It takes a specific combination of gear for this scenario to occur, but it's real.
Finally, the cheapo chinese FlySky radios have been known to trample all over the signals of Spektrum radios. It's no fault of the Spektrum radio - it's the flysky radio behaving badly... but if you have racers using flysky in your local club you might want to stay on the safe side and avoid Spektrum.