Price of Brass and Bullets mainly. Planning on buying 500 round bulk boxes of 308 to practice then reload the spent brass. Affordable bulk 6.5 CM still not available from what I can find online, hopefully price would go down in the next couple years.
Also don't want to burn out a 6.5 CM barrel before I'm good enough to take advantage of the 6.5 CM's ballistics. Barrel life of a 6.5 CM only around 1500-2000 rounds compared to 5000+ for 308 from my research.
6.5 bullets are slightly cheaper and brass is about the same price when it comes to reloading. You also won't be able to shoot out a 6.5 barrel before being able to take advantage of it's ballistics.