Well, the conversation changed a bit when hobbyking released DSMX compatible orange rx's.
http://www.hobbyking.com/hobbyking/...m_Compatible_DSMX_8Ch_2_4Ghz_TwinPort_Rx.html
It used to be that we'd recommend getting something that was DSM2 compatible and using the orange rx's, but they sometimes had issues because of the DSM2. Spektrum went and updated to DSMX a long time ago but the rx's (receivers) were expensive. Now that you can get better (much better) quality signal out of DSMX with orange rx prices, it's a lot more attractive. The 9x has it's own issues. For me, the DX6i is a simple, ready to use package. You don't have to reprogram it, reflash it, re anything it, the DX6i is ready to pull out of the box and fly. Plus it's easy to understand.
The 9x has more channels, more options, more mixing capability etc. It's also a favorite of guys who like to tinker and mess with software and reflashing firmware and things like that. If that's something you're interested in then you are hard pressed to beat the 9x.
Personally I fly a DX8. I like to take it out of the box and it works every single time. I haven't had to reflash or fix it, and I love the way the menus work. I like the programming, all the options, everything about my radio. I'm a spektrum fan because it works and if you do have any issues the customer service can't be beat. My friend has an old DSM2 DX6i that he's had for a long time. He had a gimble go bad and he sent it in. They repaired it and mailed it back all for free and it was waaay out of warranty. He's since bought a DX7s and loves it to death. He was considering the Futaba 8FG super but after getting his DX7s he only wishes he'd bought the DX8. The only reason he upgraded in the first place was he needed another channel to run his nitro heli with the governor.
Obviously we've been flying for a while and have the cash to make such purchases. For someone starting out it's really hard to beat the 9x. As long as you're willing to tinker with it a bit and watch youtube and read forums to learn how, I really don't think you'll be disappointed. If you want something out of the box solid that's easy to use, it's hard to beat the DX6i.