Well, I don't like rusty nails and fish hooks so I can understand you wanting to get those knocked out. But as far as discriminating out trash, that's a losing proposition at the beach. Because the trash signals are pretty much the same as the gold signals. If you discriminate out pull tabs and foil, you can pretty much kiss the gold goodbye too because there isn't a machine made that can tell the difference.
I had a Quattro and did very well with it at the beach. I now have an Excalibur, an Explorer II, and a CZ-70. All are outstanding beach machines. The Quattro has them all beat for depth at the beach with the Explorer being second and the Excal and the CZ about tied I would say. The Excal is simple to use, sensitive to small gold, and has a nice wide range of variable tones to help you to get to know the good stuff from the bad. In my opinion, this is better than most machines with meters. The Quattro is similar in that is had variable tones but also has a meter with numeric values and icons.
The Excal of course is waterproof and the Quattro is not, so if you get it wet it's a bad thing. Both the Minelabs are "threshold machines", meaning they have a threshold hum in the background and this can be useful in snagging targets you might have missed with a silent search machine because extra deep targets and "near misses" tend to interrupt the threshold and tip you off to give it a second look.
The DFX is also a threshold machine and I owned one for 6 months. I land hunted for deep silver, Civil War relics, and also did some beach hunting and was successful at all 3 with it. I replaced it with the CZ-70 which is hands-down better at all 3 than the DFX.
The CZ-70 is a killer beach machine and very simple to use. It has 7 LED "Notch" segments ranging from "Iron" to "High Coin" and basically every piece of metal in the world falls under one of them. It hunts silently, hits hard, and locks on to deep targets well. It's "Rain Proof" and "Spalsh Resistant" and it's also a great land machine as well.
I think that given your criteria of a reasonable learning curve and a multi-purpose machine, either the Quattro or the CZ-70 would fit the bill. The CZ is lighter. The CZ is easier to learn. The Quattro is more sophisticated and goes deeper. It gives you more info. It's a little tougher to master, but not too bad. It has a nice variable tone system and the CZ has 4 tones, one of which you won't be using (Iron) so it effectively has 3. If you get caught in the rain with the Quattro it's time to go home and probably run for the car. With the CZ you can take your time getting to the car and if you like, you can keep right on hunting. There's a CZ-70 on eBay right now that ends in 8 hours and looks like a great deal. See the link...
http://cgi.ebay.com/Mint-Fisher-CZ-70-Pro-and-SunRay-Fz1-Probe_W0QQitemZ5827334156QQcategoryZ94884QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I will recommend against the DFX for a few reasons. It's the most fragile contruction in my opinion...not very rugged at all. It's the least effective at the beach, though it WILL hunt. It's just not a real wet sand demon. It does OK, but pales in comparison to the others. And it's a very complicated machine with like 50+ adjustable settings and 192 tones if I remember correctly. And some of the settings can have drastic effects on the other settings, so it's possible to dork it up pretty good. I'm a "tweaker" by nature and into computers so I found all that techno stuff appealing at first. And I did like the DFX...I actually dug a Barber dime at 8.5" with it on my very first outing. But after a while I realized I was spending more time finger banging the touchpad and tweaking my settings than I was actually FINDING STUFF, so it had to go. That and the fact that the my CZ buddies were spanking my butt on deep bullets and relics out in the woods. It lacked the depth of the CZ-70.
Anyway, those are my thoughts. Best advice is to check out both the Quattro and the CZ if you can. Hope that helps some...