Important features to have (and use):
One feature which is important to have which lesser radios don't offer is "dual elevator". This function allows you to have the two elevator servos work together on separate channels. The reason that this is important is that it is much easier to set up the elevator if you use two separate channels. Using the end point adjustments and the trim function, you can easily get the elevators to move together. With the DX7, you must use mix 5 or 6 so that the trim will work. If you do not use mix 5 or 6, or if you have a lesser radio, when you use the trim button on the front of the transmitter to give a little up or down elevator, only one of the two elevators will move with the trim. This forces you to use a Y-harness, most likely a reversing Y-harness. This makes matching the elevators more time consuming.
Throttle curve: All 3 radios also offer throttle curves. The carburetors on gas engines are not linear at all. At 1/2 open, they are at 85% power, and little happens between 1/2 open and fully open. Some people brag about hovering a 1/2 throttle, well, that's really almost full throttle! I have the throttle stick move the carburetor quicker at the beginning and at the end. This makes the throttle more linear. The DX7 has limitations as to how the curve is set while the 10X does not, so the 10X can give you the best fix.
Exponential: is available on lesser radios, but the 10X allows the exponential to be set independently in both directions which I call "independent expo". The most important use for this is with the elevator. I use 30% expo for up elevator (on low rate) but use no expo for down elevator. This makes the plane "feel" the same whether I am upright or inverted. When I'm upright I pull a little on the stick and the plane goes up a little, when I'm inverted and I push the same on the stick the plane goes up about the same. I also have more down elevator travel than up elevator travel, like 12 degrees down and 10 degrees up, but that's available with all radios. You also need to set up the expo so that the feel of the controls is the same at center when on low rates or high rates so that you can fly the plane somewhat like "normal" when you are in 3D rates. I talk about that in "How to set up your radio".
Flight Modes: Dual Rate and Exponential are important to have. The BIG and I mean BIG advantage of the 10X is the ability to have the ailerons, elevators and rudder all go from high to low rate (and in between) at the same time by flipping just one switch, not three switches. This is a BIG advantage (did I say that already?). The ability to control all the rates at once is called "Flight Modes". To see how to set up your radio for 3D, snaps, spins, normal flying, etc. and the importance of flight modes, see the "How to set up your radio page".
Flaperons: All 3 radios also offer "flaperons" so that you can mix the elevator to the ailerons to use the ailerons as flaps for extreme aerobatics. Some lesser radios offer this as well.
Stick Adjustments: my preference is to adjust the spring tension to as high as possible and make the sticks as long as possible and use a transmitter tray. I suggest that you do the same. This will give you the most precise control.
PCM vs FM: There are arguments both ways about which is best. People who say FM is better are idiots. Use PCM. If someone sees you using PCM and says you should be using FM, tell him I said he is an idiot. Just to clarify - it is OK to use FM, it is acceptable for many planes, but it is not better than PCM. Both systems will get radio hits. When an FM receiver gets hit it reacts wildly and ALWAYS does a violent death dance straight to the ground, tree or worse. Hopefully the hit is short and you can save it before the plane impales itself into your car. PCM simply ignores short hits and uses the last good signal to operate the plane so it does not do the death dance. If the hit is a second or two, it uses the last good signal, so if you were flying straight and level, you still fly straight and level. This is what you want it to do! See below in setting up your failsafe. If it never gets a good signal again, try to sell the plane immediately, don't wait for it to hit the ground. Yelling FORE is also acceptable, but no matter what, yell something so that people can avoid being hit and can shout out a bid before it hits. Believe me, FM would not have helped in this case, it just would have done the herky jerky before impact. If your idiot friend says "I told you so", then calmly go over to his plane, jump on it several times, and then say "Gene told me to do that because you are not only an idiot but also a complete ass".
Gyro: I find using a gyro to be very helpful in learning how to hover. Connecting the gyro to the rudder is easiest if you have just one rudder servo. If you connect the gyro to the elevators, then you must Y-harness the elevators together out of the gyro, and thus you have a problem getting the elevators to move together. You must set the gain as high as possible on the gyro to get it to help during low speed maneuvers. The problem is that you cannot fly with the gyro on. The high gain will cause the rudder to go crazy in normal flight. You must switch the gyro on just after you start to hover and must turn it off when you exit from the hover. Therefore you must be able to access the gyro from the transmitter.
Setting up Failsafe: You must set up for failsafe from your radio. Set all the servos to hold their last position except for the throttle which should go to idle, unless you are at Joe Nall where you set the engine to kill. Do not set the engine to kill unless you are instructed to by the field or event you are at. 99% of the time it will come back and you need the engine running to make that dramatic save!
1 or 2 receivers? Just use 1. There is an advantage to using 2 receivers in some failure modes, but it's not worth the other problems it causes. Use 2 battery packs on one receiver. This is about as safe as it gets. Be sure to glue or tape your Rx crystal in place, as they have been known to fall out and when that happens, you are in serious trouble.