As I have said in other threads, I own a Gretsch 5120 and I do not hesitate to recommend it to other players. It's the first decent quality guitar that I've owned, and for the price I consider it to be an excellent value. That said, there is really no comparison between the 5120 and the "Pro" series Gretsches. They have different wiring, different pickups, different tuners, and are built with different woods (the Pro series are a laminate of 5 very thin layers, while the electromatics are 3 thicker layers, at least on top). Everything about them is different, and while the 5120 sounds good stock, it doesn't have "that great Gretsch sound." As I have proven to myself, it can come VERY close with the right modifications... If you MUST have the Gretsch sound, look for a 6118 (Anniversary) model; it'll get you there for much less coin than the rest of the Pro series.
The 5120 is also not an acoustic. You can play it that way (and I often do, I mostly play for myself), but it doesn't have the volume or projection of even a modest acoustic. In that vein, you might look at the Gretsch Synchromatic, which is an archtop acoustic at around the same price point as the 5120.
As Doug Smith said above, play them all if you can. Make your decision based on sound and feel first, with the pocketbook having a simple veto. When you find the one that's right, you'll know it.