Neither will be much in stock - it's already deer season or just weeks away.
First, can you shoot haji at 1000 yards? If so, then Uncle Sam already gave you a rifle, crew served weapon, or vehicle with or without wings to do it. The silly response goes to your sig line.
What are you REALLY going to do with it, besides blow money to have bragging rights at the range? Cartridges have different ballistic properties, it's entirely the point why different ones are designed.
It's been posted more than once in this thread, and a thousand others, much less the Wiki entrys: 6.8 is a 14.5 to 16" barreled, lethal to live targets design made up by Special Forces and the Army Marksmanship Unit with 40% more power than 5.56. 6.5G is a long distance cartridge designed for 20"+ barrels to excell at 600m and futher in precision range competition.
Can you use either for the other job - yes - sighted in at the same ranges, either will hit the same candy wrapper. They are both limited to feeding thru the restricted mag well of the M16, and both more alike than the fanboys of either like to admit.
Right now, there's about ten 6.8 suppliers of barrels, uppers, and ammo for every one 6.5G/.264LBC. Either way, you can get barrels, bolts, magazines, and ammo online. You will likely NOT find any at Boxmart. Ammo will be commercially priced, it's not a taxpayer supported military surplus round.
What ranges are you shooting at, at what kind of targets? Then choose what fits best.