I used to own a 70 and found heaps with it, gold, coins & relics. In quiet ground to medium you might not notice much difference. And, if you pit the MXT against it using only the DD coil on it (the MXT) you would see the 70 having a slight advantage over it on really small gold. But, change the coil like the 70 and put the 5.3 Eclipse on it and you now have it matching the 70 no probs, even maybe just beating it for sens. Then you get into the real bad ground and now the MXT SAT starts kicking it. You will or should be able to run a higher gain in that said ground and wont suffer nearly as many threshold changes, rises and drops, as the 70 will. More controllable than the 70 is as the ground gets worse. I believe better disc too in the prospecting mode.
Someone else can answer about it on the coin and relic front though as I spend more time chasing yellow than I do the other.
Besides that, the MXT's are rock solid quality wise compared to the Minelabs problem child.