I had the same problem and faced the same decision with my 960 recently. My concern was that, if it's leaking slowly now, it can only continue to erode the head gasket and accelerate the rate of the leak over time (I didn't think about using a stop-leak product). I didn't know whether the failure point would be just a large loss of seeping coolant, or a catastrophic head gasket failure when it finally went.

Given the number of other things the car needed to have repaired, I elected to just salvage the car and use my 945 for my DD.

One thing to consider, though: The 960 has the low coolant level sensor that warns when the coolant overflow tank is drained. If you continue to drive, this will provide some warning before the engine loses too much coolant and overheats. The problem is that the wire insulation on this sensor tends to corrode over time, and your sensor may not function properly in time to provide adequate warning. The sensor is cheap and easily replaced (it plugs into the bottom of your plastic coolant overflow tank). If you elect to continue driving and adding coolant, you might consider inspecting and possibly replacing this component for added peace of mind.