Ok, so let me help you, because I've got Spektrum and have been really analyzing what some of the different ones do.
DX6i - older, discontinued model. Great radio as it supports DSM2 and DSMX, but does not offer diversity antennas (which provide a better signal to the receiver), and is limited to 10 models in memory. This will also allow buddy boxing, but you can only do wired buddy boxing with it.
DX6e - current "cheap" 6 channel Spektrum made by Spektrum. This, in my opinion, is the best, in production equivalent to the DX6i, except it has more features/bang for your buck. this one has the ability to store 250 models in memory, allows wired and wireless buddy boxing, also supports DSM2/DSMX frequencies. Still doesn't have diversity antennas, but it's cheaper than the DX6 G3 (Generation 3) radios.
DX6 - This is the bells and whistles version of the DX6 line. I started with a DX6 Gen 2, and it is an absolutely SOLID radio. It allows 250 models to be stored in memory, it has diversity antennas (one in the fixed antenna coming out of the top of the radio, the other in the "handle" on the back of the radio), wired/wireless trainer link, and my favorite feature - it has voices. Now, a lot of people aren't exactly sold on this, but I LOVE this feature. With the DX6i/DX6e radios, they have alarms for things like your countdown timer, or low battery on the transmitter, etc. The DX6 has a voice that will tell you, "60 seconds remaining", and can do countdowns for you when you start getting close to running out of time. It will also yell at you when you start the radio up and your throttle stick has been bumped, "Throttle High!" Some people don't find this as useful as I do, but for me, it's VERY useful - I don't have to look down at my radio to see the timer and see how much time I have left; I can hear it.
The DX8/i/e models follow pretty much the same format and upgrades; the only major differences between the 6 and the 8 are that the 6 uses 4 AA batteries to power it; the 8 uses a rechargeable battery, and 6 has only 6 channels available, while the 8 has 8 channels.
If you can, I would recommend staying away from the DX6i/DX8i models. The lack of diversity, and the limitation of only 10 models in memory (which gets filled up faster than you'd think!) are reason enough to skip over them, unless you're getting a radio that you KNOW has never been abused/dropped, and you're getting a bargain basement price.
Even if it IS a foamie, it's still frustrating as anything to turn a plane into a lawn dart or get it stuck in a tree because of intermittent signal to the receiver...I'm sure FDS will agree. 
