I purchased both a Silpat and a SlickMat within a very short time. I purchased the Silpat second, primarily because the SlickMat wasn't available in a larger size and I wanted a larger mat. Apart from size, there isn't a tremendous difference in the mats. I prefer the SlickMat, for a few arguably superficial reasons. The SlickMat just looks nicer. The Silpat looks so industrial in comparison, with its unappealing Home Depot color scheme. It looks like something from a hardware store. I prefer the brighter white and red of the SlickMat, which seems a lot more like something you'd see in a brightly-lit, happy kitchen. The SlickMat also feels nicer -- more smooth and silky in texture.
Functionally, I suppose the Silpat and the SlickMat are on par. The first batch of cookies I made stuck to both the Silpat and the SlickMat. That's not supposed to happen, so it was a bit disappointing. Of course, it happened with both mats, so I think it might have something to do with the cookie recipe I used and not so much with the quality of the mats. I haven't had problems with sticking food, otherwise.
A final grievance: when I was opening the Silpat, it came wrapped up in a tube, almost like a diploma. In trying to open the wrapping, I actually made a very small tear in one edge of the Silpat. Perhaps the SlickMat is just as fragile, but I was surprised I could tear the Silpat so easily. Again, it's as minor a tear as you can imagine, so it doesn't affect how I can use the Silpat. But I suppose it's something worth mentioning.