I run both, sliders allow you to feel pickups quicker, but you'll find that they don't sink as fast and are a bit more snag proned. So if fishing faster water where you need to get down quick or real grabby bottoms, its usually best to run a fixed. Also run a fixed if your free spooling, a slider in this situation may hang on the bottom while the rest of your leader keeps going downstream. Ditto on the comment about bait, especially eggs, they seem to get caught in the wind and your slider will run up your main line.

But if you're fishing usual flows with reasonable bottoms and swinging through the drift, you can't beat a sliding slinky.