I'm back to distrust of teh specs again. The x700 series 2ws are supposed to have a 25" turning radius. My 455 is supposed to be 2.16' according to tractor data. Real world testing my 455 cuts a much tighter radius than the 748. Not sure how much of this is the awd, but I did the test on concrete.
This is just my take on this, and a lot of people will disagree with me, but unless your going to buy a zero turn, or a 4WS tractor, turning radius does not matter much when buying a machine. It is what it is, and if you care about turning radius, you need to be looking at 4 Wheel Steer, or a Zero-Turn.
Not much has changed on these machines in the last 5 years now. I know for a fact that all 2012 and up DIESEL X7xx series machines (X740. X748, X749) have a smaller engine in them. The fuel injection went from mechanical to electric, and the RPM's run slightly higher I believe. I have a 2012 X748SE, and the diesel is no slouch. It has a ton of power in reserve, and the fuel economy is amazing to say the least.
I have heard, but never looked into this myself, that the X748 has a larger hydraulic pump, obviously because of the All Wheel Drive system that is driven by oil, and not a drive-shaft.
Overall, AWD over 2WD comes down to a personal preference and what you need. The same debate comes up in purchasing a pickup truck. You might not use 4X4 for a year, but one day you will be stuck somewhere with no help, and you will wish you had purchased the 4X4. A good example is on the weekend, I was away, while my wife was hauling her horse trailer around. She has only done this a few times so far. At our place you have to drive through about 500 feet of muddy soft ground to get to the barn. She became stuck and because she was so used to having 2WD vehicles, she was fighting with it, rocking it back and forth. About 30 seconds later she remembered, I made her buy a 4X4. She engaged it and the truck walked right out like nothing and dragged the trailer through the mud like it wasn't even there. When we went truck shopping for her, I insisted on 4WD because I said, one day you will be in a sticky spot, and I won't be there to help you. Sure enough, that's exactly what happened.
The moral of the story is, on tractors ESPECIALLY, I always suggest 4WD, in this case, All Wheel Drive. Tractors are designed to work in mud and dirt, where AWD is most useful, not ordering a AWD machine is crippling the machines capabilities. If you will only ever cut grass on flat ground, and you intend to keep the machine for a lot of years, than 2WD might be better for you. Keep in mind, if you ever do have to sell, X748 tractors sell faster than you can get them. X740's tend to sit on the lot, because very few people want, or can get by, with a 2WD machine.
Don't let complexity scare you. The X700 Series All Wheel Drive is a very simple system with few moving parts, it's very reliable because there is no drive-shaft, u-joints, and a lot less bearings. Maintenance is slightly more expensive as the hydraulic oil changes are larger, so you will need more oil when it comes time to drain the hydro system for maintenance. Again, not enough to matter really.