I put 100k miles on my GL1800 in the 4-5 years I had it. Moved to a dual sport to get off the beaten path a little. Fell in love with the 09 yellow SERG and bought it having never even sat on a HD. I love the bike, but they aren't the same. Like comparing a muscle car to a Caddy... they each have their high points and are good for what they are, but they will never be the same. (OK, the can be close to being the same... which is why when I get home from this deployment I'm buying a Caddy CTS-V; 556 HP!!)
If I had no limits to my stable size, I'd have another GL1800... I loved riding that bike hard, fast, and long. It is an amazing touring bike that I rode for 20 hours a day for 10 days straight and didn't mind getting on it the next morning. I don't suspect I'd feel the same about my SERG.
The SERG is a beast and I also enjoy riding it; and as far as looks go, I don't think there's anyone who'd say a Wing can come close in that category.
You've got the advantage of having been on both bikes, so you've got a pretty good idea of the difference. From what I hear, the frame change on the touring bikes in '09 made a huge difference in how they feel, and the 120 is a strong motor, but it doesn't have the kick of the 1800. You're also going to have to drop a good chunk of change to equip a RG to have all the amenities of the Wing.
Coming off the wing, I didn't have any problems with the fixed fairing; as far as handling, I was far more comfortable pushing it in the twisties on my wing than I am on my SERG, but then my riding style has changed in recent years. I am sad to admit my poor SERG only has 9k miles on it. Hopefully I'll find time to change that next year when I get home.
It's a tough choice, but there's something drawing you back to the HD. If you're in a position to unload your wing and pick up a good deal on a used HD, go for it! But if you've got your wing dialed in and really enjoy riding it, I'd give a second thought to dumping it and having to get another bike dialed in.