I own and have gigged with both of these cabs, and I think highly of them both. Yes, more surface area (more cones) will definitely equal more volume (with all other factors being the same or similar). The Neo 610 is a very loud cab, and it sounds great IME/IMHO. The GK Neo410 is also a great cab, though. Both are voiced fairly deep, and both get a little lean in the mids (though you can dial 'em in). Tone profile is generally in the same range.
One thing to be aware of with the Fender is that it does not have side handles, so it can sometimes be awkward to lug around, solo.
Also, the GK Neo410 would allow you to use the 4-conductor GK cable and the HMS bi-amp option on the Fusion 550. This is a very cool option that I didn't really appreciate until I gigged it, but it lets you push the drivers a bit, while keeping the horn nice and clean.
Both are good options, IME/IMHO.