Time and again, if you read the opinions from the experts, you'll see the same pros and cons. Appearance is purely subjective, but to most eyes, the new GL1800 looks scads better than the old, but to get it to look less fat, and drop weight, too, they had to reduce luggage capacity to the point some Wingers are mad, and the GTL has more luggage capacity. Still, the GL looks prettier to many people, and is the newer exterior design, but the side and top cases are fixed and not removeable.
The Wing has a lower center of gravity, and so will appeal to less skilled and/or experienced riders (to be clear, many GL riders may BE more skilled and/or experienced than GTL riders, but the bike itself is more forgiving with the lower CG). The GL is also more stable on the superslab behind a big rig in dirty air with strong crosswinds due to bad weather, while the GTL is more nimble in the twisties, has more ground clearance and available lean angle, and tips in to corners quicker. The new GL, however, handles more quickly than the old, and if you don't try to ride like Rossi in the corners, though slower, will corner like it's on rails (good or bad, depending on who you are as a customer).
The GTL will likely lose a 0-30 race to the GL due to the torque of the Wing, but the GTL's engine will eat its lunch, otherwise.
The Wing gets most people's nods for having apple Car Play, and a bigger screen (I like the screen divided into several components [Nav, warnings and indicators, and radio/settings], but I'm a rarity in that regard). It's more tech-modern, simply put.
The Wing gets slightly better mileage, but the GTL has more range, with a bigger gas tank. While everyone notes this, I've never seen anyone who has ever bought either bike who has indicated that was a significant factor in their decision, however.
They are both excellent bikes, and are both largely capable of the same type of touring, it's just that one is a tad more sporting, while the other is a tad more stable.
Honda's serice is NOT that much cheaper than BMW's, but as you probably already know, the Wing will probably be a tad more reliable.
If these were my two options, and I had no significant other weighing in (if you want to ride with a pillion, and don't consider her opinion, you're in trouble), I'd lean Wing if I didn't mind a more leisurely ride in the twisties and planned on a good bit of interstate travel. If I planned on passing the occasional kid on a sportbike who didn't really know how to ride, I'd get the GTL. The other main consideration would be if either engine "spoke to me." Ultimately, I fell in love with the power plant, and that's how I came to own my GT.
Good luck with whatever you choose, and let us know once you decided, even if it's the Wing, and what was/were the ultimate factors of greatest importance to you.