Many of the folks over on gretschpages.com have played both; and while the 5120 is by no means in the class of the "pro-series" models, most profess to be impressed with the quality and sound of the 5120, particularly at its price point. The stock humbuckers on the 5120 are darker and not as Gretsch-y sounding as the filtertrons that come on the 6120, and the 5120 is a slightly heavier 5-ply laminate whereas the pro series are a lighter 3-ply. I'm sure there are other construction differences, but those are the biggies.
It is pretty simple, and not too terribly expensive, to make a "budget 6120" out of the 5120, I've done it to mine with a custom pickguard and jackplate, rocking bar bridge, grover locking tuners and TV Jones pickups. My total investment is around $1100 including the guitar, parts and labor; half of what you would pay here in the States for a Pro-series. I haven't had the opportunity to do a side-by-side sound comparison, but I think mine sounds awfully good.
"There's such a fine line between genius and stupidity."
--David St. Hubbins