i going to agree with most of what Sharp said and impedance being the first factor to look into....and Ohms. this will determine the sensitivity and volume of your headphones...and the power needed to drive them.
sound is my business and ive been a soundman for 40years
after that.... they need to be comfy... you are going to be wearing them for hours...are they too lose and constantly adjusting them on your head... too tight, maybe digging your sunglasses into the side of your head...lol i hate that one
isolation is another factor... how well they will block out wind and external noise so you are hearing your signals clearly...(who cares about that approaching train or boat.. i want to hear GOLD)
how easy you can clean them... they will be in dirt or sand (like mine) and that goes into durability realm as well...
waterproof?? mine are...so cleaning them out is easy... but most are not and even sweating in them over time... destroys them.. i use $350 headphones at work... and destroy a pair every year... sometimes more... just nature of the job and price i pay because i love them and i can wear them 12hrs a day. but they would suck for metal detecting and im lucky my whites headphones do the job for me and are waterproof. but if they killed my head... i would cut that cable in a minute and put something else on.
which brings me to my last factor.... price!! in the case of what we do... high price does not mean better. 99% of the headphones out there will go from 20-20k and bet most of you cant even hear that well
so dont buy something based on price... if a $20 pair of headphones does what is needed for you... thats great !!
my whites headphone are waterproof and dont sound all that great and cost like 150. but they do the job for detecting and are comfy enough for me... so ive stayed with them and never had a problem...i need to go knock on a piece of wood now...
good digs