Electronic locks are nice, they are quicker and simpler to use. However, if they go out, you are stuck with a safe you can't get into. Which means drilling the door, and then installing a new lock.
Dial combinations will probably last forever, though you should change the combination once a year to prevent wear on the internal parts.
Ask me how I know all this.GI9
Updated:
I have one of the Costco Cannon made safes. It has worked very well for the past 8 years, until December when the LeGard electronic lock went out.
Fortunately, the lock is covered under a lifetime warranty by Cannon, and they replaced it with a manual at my request.
Safe cracker said that Cannons are decent safes, and have good fire resistance. However, LeGard locks are not good locks, His recommendation was an S&G lock(Sargeant and Green), which is what Cannon had sent me as a replacement.
I have no problems with the Cannon safe, other than it is getting too crowded, so I need to look at getting another one.
I won't get one with an electronic lock. It took 2 hours to drill, and replace the lock, and cost $370. About 2/3s the cost of the safe when it was purchased.