How well you know the used one, that is has it been taken care of, do you know the person selling it well enough to know there is no problem with the used one you will find out first time it goes into the water and it leaks. How old is it and if the battery holds a good charge. A new battery is over $150, if it has to be sent in to be repaired because of a water leak or other problem it looks like they have been from $200 up to repair with some I have read over $500 to repair.
with a new Excalibur II you have a brand new Nickle Metal Hydride battery pack which last longer than the older Ni cad packs, it has the new lighter weight Tornado coil also most like more than the older BBS coil and you have a warranty too so for the first year you don't have to worry about paying big $$$ for repairs. I believe the warranty on the Excalibur's are only 1 years while the other Minelabs are 2 years. Looks like the selling price are around $1200 for a new one versus the $700 for a used one plus if it is a older one like over 10 years old and needs repair they may not be able to as they cant get parts for some over 10 years old. Something to think about isn't it??