For the original poster, and the point on which way to upgrade .... I guess there are a number of ways to look at it:

1: - If monies no object, get whatever high end Futuba or Specktrum/JR 2.4gig system you feel like, they are all good quality systems with positives and minus's, PERIOD!.
2: - If like some of us, your careful which way you spend your hard earned money, perhaps the best route would be either that new JR 11 channel 'transformer' looking thing, or even a Dx8, as then you can sell or buddy box your Dx6i (re friends flying your planes etc) as you'll not need to buy new receivers for all your existing DSM2 equipped aircraft, just re-bind and re-program them to the new TX (thus saving you some money).
3: - The other bonus, is that if you stay with Specktrum or JR, you won't need to re-understand the expo setups of a new brand, as I understand Futuba have differing levels of values to Specktrum/JR (expo etc), even more so when talking heli setups - like I fly. (someone correct me if I'm wrong, I only have Specktrum, and was told that info second hand).

At the end of the day, you should be right with any of the leading brands high end units. I'd just take into account what is used at your club (if you belong to one), as they normally have a more common system that most folk normally use (buddy boxing deal again should you wish to help someone into the hobby), and at worst, it'll mean that specific brand should be better supported locally at the LHS level should/when you need it!

Myself, I run Specktrum with 6volts at the RX, and have NEVER had any issue's ...... yet I still like the look of some of the Futuba stuff have out now days. At the time I got my system, I just went with the one that ticked all the boxes that I had set for myself ...... to hell with that whole 'chev vs ford' mindset that someone mentioned, all it does when your totally focused on one thing is prevent you from looking at the wider picture that's going on around you ...... I guess that's why they call it tunnel vision, as all you end up seeing is that train light just prior to it smacking you upside the face at 100mph! (thus why I TRY to stay out of tunnels)