Those are all air tests in that link, who cares. Once your coil hits real ground a whole lot of factors are going to happen. How much mineralization ? How well are you ground balanced ? How much trash ? How well do you understand your machine ? You are never going to get as much depth in the ground as in the air. Don't believe all those goofy depths you hear people saying they found a dime. It just doesn't happen. Larger objects are a different matter....but digging a smashed soda can at 18 inches isnt fun.
On to the next situation. Weight. If you put a bigger coil on the machine, it ups the weight signifcantly. A pound or so may not sound like a lot, but comparing a 5 pound machine vs a 3 pound machine for any length of swinging time makes a huge difference. If you don't have a lot of arm strength, you want a lighter machine or you will be sorry you bought a big boat anchor you will never use. You may be able to swing a 3 pound machine all day, but at 4.5 pounds you wear out in an hour. On top of that, the big coil is going to pick up all the trash and mask the good targets nearby. Big coils are only good for one thing...covering a lot of cleanground fast. Use it on the beach or a park, your setting yourself up for a lot of frustration.
The DFX is a great, top of the line machine, don't worry so much about depth to begin with. Just learn to use and understand whatever you buy (just DON"T buy an offbrand machine!! Ask if you don't know if it's a decent brand or not).
Lastly, ask RiverRat to give you one of those nice walking sticks to beat off the guys who stare unless it's me. They look like they would be pretty painful.