Slim rack battery if available.
Bottle batteries tend to have low capacity and therefore range, the bottle mounting points are not designed for much weight and many have had stability problems with batteries mounted. Also a locking arrangement to prevent theft isn't easy to provide for a bottle battery
Conversely, rack batteries are often quite adequate capacity, can be lockable, and rear end weight doesn't have much effect on handling and is good for traction. If stealth is wanted, panniers can do a good job of masking the battery, making for more stealth than a bottle battery.
So for me it's no contest.