I think if you're beating steel to death at distance, the 6.5 is the ticket. The exterior ballistics are just better. Less drop, less wind drift.
BUT - to get that requires a decently long barrel. I don't know how it does if you chop it down to 16" (like the FBI is doing to all their rifles) so you can add a can and not have a ridiculously long and heavy rifle. Speaking of barrels - It burns them out at a much faster rate. From what I hear, there is no comparison. The .308 will last forever comparatively. If it's a work gun - try telling your admin that every few years you'll need a new barrel compared to the .308 that will last 2 careers.
How does the 6.5 do in gel through intermediate barriers? Are there barrier rounds available? What does the supply line for whatever round you find going to be? Can you get enough to practice with a lot on whatever your budget is (I think in terms of PD buying ammo).
Are you talking about a work gun? Let's be honest - most sniper shots average in the 50 yard range (per ASA). How much better ballistics do you need to shoot across the street? And 500 yards is so stupid easy with a .308 and good boolits, what would be the point? Plus, I know how my .308s perform with our open air round and barrier round (we just call it a glass round, because I don't see us shooting through stuff we can't see - even though we technically could).
What you want to use it for totally drives this train. If you want to stretch out, play at long range and murder the shit out of steel targets way out there - the answer is clear. The 6.5 CM. If you want to shoot bad guys at cop sniper ranges, the .308 for now. Always subject to change, but I'll take something that I know works and I know there is adequate supply at a decent price.
PS - I want a 6.5 CM, but can't make myself buy one since I get free .308 and my everyday range only goes out to 500. I can set up a 950 yard range in an afternoon, but that also isn't really that hard with a .308 either. So unless I can really stretch it out somewhere reasonably close and reasonably accessibly on a regular basis, I'll keep buying unnecessary crap for my jeeps (I actually own 3) instead of a new rifle, ammo and optic.