I have used both. Here are some thoughts on the subject:
GX617 lenses are a lot sharper, at least in my case.
GX617 viewfinders are actual finders not like the window in G617. Having said that both are almost equally inaccurate. It takes practice to visualize what the actual picture will be. Probably slightly better on the GX.
G617 level visible from the view finder works very well both when shooting handheld and on the tripod. I have added a level on the bracket above the lens on both GX lenses for the same reason. The level on the GX617 is only useful when you want to place the camera absolutely leveled both horizontally and vertically.
Film door seems a bit better on the GX but not so when on tripod (which in my case spends most of the time) as it interferes with head controls. Different design head may fit better.
GX requires batteries only when you need to use the button on top of the camera. On the G617 is mechanical but on the stiff side. I usually use a cable release on the lens on both cameras.
I don't know who the genius was, but the GX has no accessory shoe. The G617 has two.
Both film door opening, and film release switches are much better designed in the GX.
Having said all that, in my opinion there are two reasons to switch to GX: sharper interchangeable lenses. Everything else are just nuances.