CCT is correlated color temperature, often just shortened to color temperature. Higher is more blue. Lower is more yellow.
Flats are parts of the battery tube that are ground flat instead of rounded. This doesn’t provide any meaningful objective advantage or disadvantage.
The XP-L HD produces more lumens than the XP-L HI. The XP-L HI produces more candela (throw / beam distance) than the XP-L HD.
Which color temperature is “more beautiful” is a matter of opinion. In the opinion of the average BLF member, 5000K is preferable to any other option, and very preferable to 6500K.