The two that I built and use does neither. I use the router table fence as the guide. When running the part through, apply pressure towards the fence and this will give you a straight cut every time.
IMHO, keep the design of the coping sled a simple as possible. My first couple versions did in fact use the miter slot, and I found it caused more problem then it helped solve. It is MUCH easier squaring up the fence of the coping sled to the fence of the router table then squaring it up to the miter slot.
Also I do have one that I use for general cuts, and will occasionally need to change the fence on it, as it helps prevent tear out. I have a second coping sled that I use strictly us for rail and stile copes so that the fence on that never needs changing, and gives me something to use to set the height of the bit quickly.