both can be seriously overused. as (i think) heath said, don't dissolve from one shot to another, gets really old and is amateurish.  I rarely use dissolves.  I only use fades when there is a change in location or a passage of time, and even then I rarely use them.  Don't fade from shot to shot to fit the vibe or to make something seem epic, it never really works; believe me, i've tried.  i prefer straight cuts from shot to shot, it seems cleaner to me and it takes more skill if you're cutting between locations or making multiple cuts for the same trick.