Snap ring and here is why ...

When you tie directly to the eye, the knot can slide to one side or the other when you cast. If the knot is off to one side the lure will run crooked because it is being pulled off center. The snap acts like a hinge and lets the lure swing back and forth freely. With a little tuning as described here:

http://www.msslammer.com/caretuning.html

You should be able to get any Slammer to run straight even at high speeds. If you have one that kicks randomly to the side every so often that is even better. My two favorite plugs both have this wild kick to them. It looks like they are going to run sideways but then they will come back and go the other way. That's when you know you have them tuned right.