Lithium (or something similar yet to come) is the future of the power source for detecting simply because of the performance and weight issues demanded by detectorists today. The real issue will be whether they are built in like the Deus/Equinox or accessible like the CTX/Etrac or RNB units. The Deus batteries require real disassembly of the components to replace including cutting the bottom out of the LF coils. Shipping and handling and 50 bucks on a 3 component WS4/5 unit is probably going to run you $170 altogether. 4 years into my Deus' lifespan the batteries were good for 2/3 hours before the warning symbols began flashing , who knows on the Equinox? Top level lithiums seem to have about 1000 cycle lifespans , so you can do the math.
The next thing to consider is availability and the key element in that consideration is not the battery itself but the connector each manufacturer uses. Without the correct connector on the battery it is useless unless you are going to cut the connector off of the old ones and want to solder it on a new battery or if you can buy them pre-assembled. With the Deus it is not so simple to find or inexpensive. Again we have no idea , yet , what the "correct" battery/connector combination for the Minelab will be or if they have designed it so only they will be able to do it.