68gr is a Hornady bullet, and 69gr is a Sierra bullet. Both are premium bullets, but a rifle will show a varying degree of preference for one over the other. This is highly individual to the rifle, however in my experience Sierra Matchkings almost always work well, whereas sometimes Hornadys can be a little more particular. My older PRI Mk12 upper would shoot Hornady 75's into no bullshit golfball sized groups at 200, whereas my particular Stealth can barely hit a playing card at 100 with them. Every precision rifle I've owned will shoot SMKs into tiny groups.
You'll have to order a sample of both and see what your rifle prefers. Also consider adding FGMM 69gr to the mix, and BH 69 TMK. You'll likely find the FGMM 69 to shoot a hair tighter but the cost may not be worth it, and that the 69 TMK bucks wind better but may shoot groups inherently a tiny bit larger than the corresponding 69 SMK. Again, you won't know until you see how they shoot out of your individual setup.