The short answer: get the MC7750!
Probably the biggest improvement isn't the raw download/upload speed of the unit, but rather the reduced network latencies. All the advertisements are only talking about the improvements in MBps, probably because it's the easiest thing to describe to people in 30 seconds or on a billboard. In reality MBps only matters if you are watching video, or downloading large files. Most of the delay time for other applications comes from the several hundred milliseconds it takes the computers to make a round trip communication. Each time you click something on the web, there are several rounds of communication which can add up. If your current connection has a 400ms delay (as mine does) it takes several seconds each time for anything to happen. Some better connections like HSPA are supposed to have about 100ms latency, but LTE is going to push that down to about 40ms. Can you tell that I'm excited about getting LTE 
I'm planning on getting the MC7750 as soon as Lenovo releases it. Unfortunately, every time I call in, I seem to be told that it's just a month or so away. I've been hearing that line for almost 4 months now. I was also told that I will be able to upgrade my existng x220, which had better be true otherwise I'm going to be royally pissed because I bought the system with that understanding.
You may find my thread on the MC7750 for the x220 device interesting.
FYI: If you want to find out the latency of any particular connection, type:
ping google.com
and get a ballpark average of the numbers. I did such a test on all the carrier's systems, and I found LTE to be the fastest. You can also find that number if you go to http://speedtest.net/ which also gives you the above mentioned MBps.
