You will want a QD system on the ball-head's picatinny interface, otherwise the combination of rifle and ball-head becomes very cumbersome to mount/dismount on the tripod. The biggest problem that I find with the direct-mount option is that there is no picatinny rail at the balance-point of my two rifle chassis. Ideally, I want the rifle mounted right in front of the mag well. However, the picatinny mounting points on my rifles are further forward on the underside of the handguard. As a result, direct-mounting for me ends up with the rifle very butt-heavy and unbalanced on top of the Tripod. You guys may have better luck with different stocks or chassis systems. I am using a Cadex Guardian .308 and a Rem 700 in an MDT ESS chassis.
Another option is to direct-mount a HOG or PIG Saddle to your tripod head and forego the ball-head or levelling-head entirely. Depending on how much tension you apply, there will be enough play between the rifle and Saddle to allow vertical adjustment. The tripod's rotating head will provide you with horizontal adjustment. It is not as refined as using a ball/levelling-head, but lots of folks opt for this system because it is simple, less expensive and lower-profile.