The 9x is a GREAT transmitter. But it's stock RF system is a HORRIBLE choice for multirotors. Thankfully it has a module bay so you can swap to a different RF system...but the antenna for the stock module is hardwired so you have to open the module, unsolder the antenna, remove the module and antenna, then remount and resolder the antenna if you want to keep using the module but actually be able to use other modules. It's not hard, but it is some fine soldering.
The 9XR is basically a 9x that's been upgraded with a backlight and USB port for programming but doesn't come with an RF module. That way you're getting a TX that's got the upgrades most people do to a 9x already done and you don't have to deal with the limited stock RF module.
The problem with the stock RF module is two fold:
1) A lot of people find it unreliable. Personally I haven't had any issues with it, but a lot of people say it's just not reliable and will loose link randomly.
2) None of the commercially available RX's for it have any kind of failsafe which is a MUST for a multirotor. Combined with the specter of #1 and bottom line is the "flysky" RF system that the 9x comes with is completely unsuitable for multirotor use unless you want to watch your nice new multirotor suddenly fly away by itself. (Note: Always put your name and contact info on the multi where it's easily visible and weather proof just in case it does fly off - that way you have a fighting chance of it finding it's way home. I would have lost my first quad if it wasn't for me having my name and phone number on it.)
Now. The issue with the FlySky RF system can be overcome if you build your own RX. I know that sounds intimidating but it's really not that bad. You only need about $7 worth of parts and the ability to solder about 6 connections. This is what I use on one of my quads and it works quite well.
I built my first quad for about $100 but I used parts I had on hand to make my own flight controller out of an arduino mega and an old wii controller. I have an article about it up on the main site and a thread here in the multirotor forum, click on my name and search my posts for lots more details.
There are lots of good parts to recommend...too many to list right now without knowing a bit more about your goals. So instead I'll share a few things experience has taught me to avoid:
1) FlySky RX's - no failsafe.
2) Multistar ESC's - the new v2's run blheli and are probably not that bad...but the originals really aren't good for multis at all (Again, I have a post with details about this if you search my posts) there are many better ESC's that are equally cheap and can be flashed easily with SimonK or BLheli...and there are more and more options available pre-flashed.
3) Nanotech batteries - They cost more and are supposed to be better...but I've been having more and more issues with them and am never buying them again. Pretty much every one I've ever owned has gone out of balance FAST even when babied. The plain "turnigy" batteries from HK last longer and stay in balance longer plus they cost less.
Other suggestions:
Learn to solder and budget for a nice soldering station. Even for $20 you can get a decent station now so there's not much excuse not to own one. It's a useful skill and isn't that hard for most people to learn. It will save you more money down the road and open up way more options for making your build(s) better.
Get a usbasp flashing tool and don't be afraid to flash your own ESC's and flight controllers. It's not that hard once you get things setup and can save you a lot of money by buying cheap generic ESC's and reflashing them instead of buying pre-flashed ESC's. It also will let you keep your flight controller updated and can be useful for things like reflashign OSD's and other stuff as you progress in the hobby (Can even be used to flash custom firmware to that 9x if you go with the 9x instead of the 9xR)
Build small and light to start. Bigger heavier quads crash harder and you spend a lot more time repairing when you crash...and you will crash. The FT Knuckle quad is a great mid-sized that's not as big as the 450's but bigger than the 250's so it can carry more stuff and run bigger packs for longer flights...but is still light enough it can crash with minimal damage. With zip tie mounted motors most crashes will only break a prop or two and maybe some zip ties. When you start flying harder and crashing harder you'll start breaking a few wood booms which are easily replaced since you can make 3 from a $2 piece of wood.