It really depends on the location, quality of the mooring/ slip and what your use is. It is typically easier to access a boat from the slip, but cost are typically higher.
Both have a potential for damage to elements and other boaters and simple theft. Location and security have wide variations.
I personally avoid slips when ever possible and only go into one on very rare occasions preferring mooring and anchoring when I'm to be aboard.
I prefer a sailboat to a motorboat, and it is my belief that boat sailing is a finer, more difficult, and sturdier art than running a motor.... Jack London