The key difference with the Live is that is has 16GB onboard storage for samples. If this isn't enough, you can expand storage further with a 2.5-inch SATA SSD/HDD. Most importantly, you no longer need to connect it to a PC/Mac and use the Live with the MPC desktop software, though you can if you want. The MPC Live also has a built-in battery, making this a truly portable, standalone music production center (the C, in MPC, previously stood for "controller").
If you're less worried about making music on the go, or just have more free desk space and cash, the MPC X is a more fully-featured take on the MPC. The core features are very similar -- 16GB of expandable storage, and it's a standalone device. The main upgrades are a 10-inch touchscreen, more rotary controls, additional MIDI ports and CV connections that allow the MPC X to control older, legacy gear.
The big take away here is that Akai had been trading on the MPC's legendary status for a few years, and perhaps losing sight of what the people that love it really wanted. Now, with competition coming from the likes of Native Instruments and even DJ stalwart Pioneer, it looks like Akai has decided to come out fighting to maintain the MPC's legacy. Both the MPC Live and the MPC X will be available around March costing $1,199 and $1,999 respectively.