I am of the school of use them till they break, then upgrade/enhance when necessary to repair.
Of course, I carefully maintain the detector being used and have a back up detector.
As long as both detectors are suitable for the domain which I am hunting (dry sand, wet sand, shallow water) I usually treasure hunt with the weakest/least capable detector, so that when this detector fails there will be no decrease in performance or capability.
Failures in Metal detectors are usually catastropic and there is little to tune.