I have owned and shot both.
The 6.5X284 is a very nice long range cartridge, but it is very hard on barrels. The 284 case is also almost too long for a short action, particularly with VLD bullets. Its true accurate life span is about 1000-1200 rounds. It will open up and cease to be a true precision round after that.
The 140 Berger VLD and the 142 Sierra Match king are high BC bullets with great long range performance and both work very well in the 6.5X284. SPeeds of up to 3000 fps are certainly possible, and before the advent of ultra high BC 7mm bullets. it was really the cartridge to beat at 1000M.
The 6XC is a cartridge based on the 22-250 and uses a smaller bullet. It is capable of pushing 115 Berger bullets to velocities near 3000 and can easily push 105's over 3000.
There are pros and cons to both... the 6XC having longer barrel life (2000 rounds or so) and a great selection of 6mm match bullets easily fitting in a short action.
the 6.5X284 has better brass (Lapua) and is a proven long distance performer at the cost of barrel life. While it will fit in a short action, removing a loaded round with a VLD bullet requires removal of the bolt.
I am a huge 6mm fan. If you can get 6XC components it can work very well, but so can the 6.5X284.
Ballistically the 6.5X284 has a better long distance record, but with poor barrel life. The 6XC is very accurate, but lacks a selction of good brass and dies.
I would personally advocate a tight-necked 6-6.5X47 over a 6XC for its brass and smaller primer pocket.