The Solo is 3750$ and the Advantage is 2845$ (for the fitted version)
There is the description of each saddles on Custom Saddlery website and they are really fast to answer any questions you could ask.
But mainly, the Solo has buffalo leather and is also monoflap with the V billets that can be adjusted. They say on the website that these saddles are good for horses with a broader back. It is custom made.
The Advantage is an 'off the shelf' saddle. Plain leather and not monoflap. It can be fitted for more $. The website says that these saddles are good for high wither horses.
My WS is off the self, has plain leather and was $2495. You need to ride in them both to really know what the difference will be to you. I think that I did when the rep came to my place, but I honestly can't tell you anything other than I loved the WS (narrow twist, big sweet spot, short block didn't get in my way) and all the others were "kinda bleeh, just doesn't fit".
In my experience, the wolfgang is better suited for a flat backed horse; the advantage for the curvy backed horse. The adv will also accomodate a horse with a higher wither better.
For the rider, I find the wolfgang's twist to feel narrower. There is more freedom, particularly in leg positioning, with the short block wolfie than with the advantage.
The Adv is more like a baseball glove/couch (in a GOOD WAY) for the rider.
I've owned both and competed in both.
Both are available "custom" with the upgraded leather through the custom site/rep or in off the shelf models (though Dover/Dressage Extensions) with less expensive/slightly more slippery leather. The off-the-shelf leather is fine (not like Stubbens of old or anything) unless you really want the buffalo upgrade, which gives you a tackier ride.