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I own the original Sonatina (Avalon style cabinet) and have auditioned the Gallo Ref 3.x (earlier gen, not sure exactly which) pretty extensively. These are not the specific models that you're inquiring about, but earlier versions of each. With that caveat, I'd say of the earlier models that:
The Gallo has more bass output, is significantly more dynamic, and had a more "open", boxless sound than the Silverline. The Gallo tweeter is a very wide dispersion example, which may account for the latter, but may also be a concern in smaller, lively rooms.
The Silverline is a more "polite" sounding speaker that really shows intimate, acoustic music to good effect. It's not at all "warm", so it provides a nice combination of transparency to the chain ahead and musicality.
IMHO, this one is apples and oranges. As noted, I can't say how much this carries over to the more recent variations that you're considering.