Well, given your criteria, it would seem a no-brainer to me. The Excalibur is a water machine. Yes, you COULD land hunt with it and many have. But that isn't it's cup of tea. It's a killer beach machine though and I have probably found 50 rings this summer and a 18K 26" chain with heavy anchor & eagle charm that's worth close to $1000. Not to mention a couple of hundred dollars in clad. That said, the Quattro definitely has the depth over the Excal at the beach. I would say it has as much as an extra 3 to 6 inches. I've dug rings and small coins at a foot or more with the Excal. With the Quattro I have dug many targets at 18" plus. I've got a good signal off of a quarter once with the Quattro that two CZ-7a Pro's got NOTHING off of, at max sensitivity. My buddy pulled a dime while using my Quattro that my Excal got NOTHING off of, not even a null. And it was cranked up too. BUT, the Excal is waterproof and the Quattro isn't. So when I work the water's edge at low tide and work the surf, there's a danger of splash and dropping. And more than once I've been hit by a rouge wave. The thought of trashing an expensive machine was too great. And if it rains, I hunt on. And the Excal is plenty sensitive and goes deep enough for me for the beach. I've done VERY well with mine.
Now, for you, since you are going to be doing SOME beach and also doing relic and silver coin shooting, I think the Quattro is the clear choice. As long as you won't be doing too much IN or real close to the water. The Quattro is killer on Civil War bullets. I've done lots of hunting in the woods with it and it punches down deep and locks on good. Minnie balls, musket balls, buttons, silver coins...very good. And wheras the Excal will give you tones only, you CAN tell a quarter from a dime from a penny when you get to know the tones. I can. Not everybody can though. But with the Quattro, if it's a 38 IT'S A QUARTER. If it's a 37 it's probably a dime. If it's a 30, it's a musketball and maybe a 32 a minnieball. So with the target ID AND the tones, you get much better ID. If you are very good with the Excal tone system (or if you just want to dig everything) then you COULD hunt effectively with it on land. But if you want superior target ID on land, killer depth on the beach, and a machine that's a little better balanced and built, then I think the Quattro is the best choice for you.
Personally, I like having a dedicated beach machine and a dedicated land machine. And a backup that will do both well. That's why I have an Explorer II for the land, the Excal for the beach, and a Fisher CZ-70 for an anytime backup or switch-hit machine. In answer to your question as to if I had to choose JUST ONE between the Excal and the Quttro to be my ONLY machine, Quattro.