Years ago your question
Would have been relatively easy to answer but not today. There is almost no end to the number of capable detectors out there it seems.
Of course the newest on the market is always the best until another "best" comes along so beware of marketing hype in choosing the detector that fits your needs.
While almost all will find stuff you might consider the following:
WEIGHT:
If you throw hay bales, do a lot of pushups, practice gymnastics, rope climbing, or otherwise have no shoulder or back issues the fact that some detectors can weigh 5 lbs or more might not mean anything to you but... if your rotator cuff won't rotate? or if your back won't get back in action? You might want to look at the 2 pound model.
COMPLEXITY:
Detectors today can have more computing power than Buzz and the boys had to go to the moon with or.. some just beep which allows you to free up your mind while hunting to look at the cows in the pasture, scenery on a beautiful day, or keep an eye out if you're hunting on the wrong side of town.
Today's high tech computer wonders can tell you where you are on planet earth with screens and numbers and squeaks and whistles that would make r2d2 envious. They will tell you how deep, how big, if it's iron, silver, kryptonite, or the hubcap from a 1952 Desoto. I've not heard of anyone walking into traffic or off a cliff that I know of while trying to interpret what these types of detectors are saying but I suppose it's possible.
One thing they won't do though is tell you whether or not that 900 lb cow behind you has balls. You have to figure that one out on your own.
So, Beeper or Starwars?? either one will get the job done.
COST:
I don't think in 20+ years of detecting that I've ever seen used detectors priced as cheap as they are today. You can buy a really great machine for cheap for sure and I would really suggest looking closely at the opportunity. Or...
You can choose from a new machine moderately priced around $300 bucks plus to one that you might have to put your house up for to get a loan to buy. The choice is strictly yours but try to remember that a good used beeper is a stepchild to past detectors that have made all those great and wonderful finds over years past.
Personally? I still like to be surprised when the target comes out of the ground rather than looking at a picture on the screen of maybe a 1944 war nickle with a mis strike on the date? but that's just me.
In closing? I truly believe an Octopus would make the best detectorist of all time!! Most of us probably have more detectors than we need but who else could actually swing eight of them at the same time?? Just sayin...
