The X2 is the older very stable tuner it holds 3 tunes, datalogs, reads and clears DTCs.
The X3 is SCT's newest flasher that has more memory (added just for '07 and up Mustang GT Autos and similar vehicles where the file size is huge), has a slightly larger screen, but does the same thing as the XCal 2 otherwise - it has not been around as long and in some applications some dealers report it as less stable. It also has 'canned' tunes on it but can be custom programmed. I have sold a bunch of them and they were all OK.
Neither will act as wideband by themselves and get accurate WOT fueling. BOTH will be able to get accurate fuel trims at idle and part throttle since they are at stoich and the that data is derived from the stock 02 sensors.