Ok, I'm "in on this one".
I hunted with the Quattro on land and at the beach and found it to be an excelelent machine. It got me some silver coins, bullets, and buttons at several sites I had pounded with my other machines, so it proved it's worth. I took it to the beach and dug the deeoest targets I've ever dug and got both gold and silver rings with it. I believe it's the deepest wet sand machine there is but I can confirm it's deeper that a DFX, a CZ, A Sovereign Elite, or my Excalibur. That said, I do use the Excal for the beach since dropping the Quattro or the Explorer II in the water would kill them.
So when I got rid of the Quattro and got the Explorer II I did the same things, soing to my old sites, going to the beach. And my results were the same...I got a few targets I had missed with the others, including the Quattro. But VERY few. And as Beachguy said, I believe the QUattro has a little depth on the Explorer at the beach and the Explorer II has a little depth on the Quattro on the land. A little. Both are great at doing either. The Quattro is easier to learn and use right off the bat, HOWEVER...the Explorer II is NOT hard to learn and use quickly. What's a LITTLE hard is to MASTER it. But that too is quite "do-able" between all the great help on thse forums and several KILLER web sites.
Which is right for you? Well, if you're set on one or the other I would say the Quattro if you will do more beach hunting or hunting in woods or open fields and relatively "clean" areas and the Explorer if you will be wanting to work more in trashy areas or tough ground. The Explorer does better in the presence of multiple targets, in my opinion. The recovery speed on the Quattro between targets is slower. Both machines excel in their elements and both are true multi-purpose machines. These are my opinions, relative to use. Others may not agree. It should also be noted that I run both machines "hot", choosing Manual sensitivity over "Auto" and cranking things up.
As Marc said, the Explorer II has a dual-digital readout mode in addition to the "Smartfind" mode. On the left of the screen (In digital) is the "Ferrous" number and on the right is the "Conductive". And the scale is 0 to 31. So a rusty nail might have a reading of 31-00 and a silver quarter maybe 00-29. So if you had a tone that was telling you coin and the number was 00-29 you would be digging. But if it sounded maybe good but the reading was 31-00 you would probably move on. I like that. It works. For a land hunter and for somebody that likes to TWEAK and have lots of control the Explorer II might be the better choice. But for the beach hunter or woods hunter that wants to turn it on, crank it up, and forget about it...the Quattro is a winner too.
So that's it...my two "Large Cents". I will leave you with links to a couple of sites I am fond of and a couple pictures I took in the field.
http://www.bdomineau.com/index.html
http://groups.msn.com/MinelabExplorerXSusers/digitalchart1.msnw
http://usetheminelabexplorerlikeapro.blogspot.com/
http://www.frontiernet.net/~jvokes/detecting.htm