What are the differences between a phone of 30 years ago versus now...every model improved in some way...but I still like using my Bell telephone with a rotary dial...it has the best sound quality of any available phone...
back to Minelab Pi's...the 2000 was the first and opened up the modern pi gold rush in Australia and here. The 2100,2200, GP Extreme, 3000, 3500, 4000, 4500 and now the 4800 and 5000...each one improved some things in the tuning and sensitivity areas. Every one is "better" than the previous model...now the 4500 and 5000 are touted as being extremely sensitive to sub-grain nugglets ( goldbug stuff)...if that is what you want from a 5500 buck detector and you have the money...go for it.
Hot ground, Emi, descrimination, ease of use and sensitivity are where the newest minelabs shine...for depth on gram or bigger nuggets I doubt much has changed in detection ability, the ability of the detector to sort the gold signal from the ground matrix is a different matter....
this is getting too long...........
as was mentioned there are repair issues to beware of...check with Minelab before you buy a used unit...also there are Modders with spare parts you can find...
In case you are wondering, I have used the 2200, gp extreme, currently have a finally fully functional 3500, have used the 4000 and 4500...anyone of them has found me gold...it is not just the machine....
Now, my unasked for suggestion...
if money is no object buy a 5000 and learn to use it like your favorite toy...or, buy a good used minelab and practice, practice, practice...or, if you are into beach hunting, coin hunting and occassional nugget hunting consider the Whites TDI Oz version...if available...yes, I know they say you don't need the expanded Gb, but you may and you may want to stop by Oz on your way to ShangHi...who is Whites to limit your options??? Oops, there I go...
anyway you asked, I answered and I hope this helps...
merry christmas
fred