I use 'em all, so, my two cents:
Everything has roughly the same video quality, certainly within a margin of color correction, so that's likely pretty irrelevant to most.
Ghost Pros:
- Great form factor. Doesn't catch much wind on a helmet, the remote is superb and battery life is excellent. You can also carry spare batteries and swap them quickly and easily.
- Very nice LCD and menu system. Quick and easy to format a card, access settings, etc, and you can do it all on the camera. It is by far the best in the field in this category.
- Rotatable lens, makes it easy to level on different helmets-- a big issue for me.
- Least annoying mount solution, but unfortunately also the least flexible-- it's only really good on a helmet.
Ghost Cons:
- As mentioned, mount solution really only works well on a helmet.
- Setting the angle on a mount is a bit of a pain-- I added some reference marks to mine so I can quickly switch between helmets and have the correct angle.
GoPro Hero 4 Pros:
- Largely indestructible in a housing.
- Lots of mount options.
- Good battery life.
- Great video quality.
GoPro Hero 4 Cons:
- Crap form factor. I can't believe so many people put up with that brick on the side of their 'lid.
- Can't quickly access the camera if it's in a housing.
- Garbage audio.
- Remote is crap.
GoPro Hero 4 Session Pros:
- Awesome form factor. It's the size they should have made the thing to begin with. Makes it very easy to use as a "B" camera in all sorts of weird places.
- Superb video quality and many options for wide/medium field of view.
- Decent menu UI.
- Audio is better than the Hero 4 due to diversity mics, but still marginal in wind, which on a bike is almost always.
GoPro Hero 4 Session Cons:
- Need to use the phone app to do anything meaningful so far as settings go. This is a pain at the side of the road.
- Battery life isn't awesome, especially with wifi on, and no option to replace (battery is fixed internally).
- As with the Hero 4, GoPro's remote is a travesty of fail.
- No external audio input.
I haven't been playing with the Sony X1000 long enough to opine on it well, but so far it's a solid little camera with superb video quality-- it may replace the Ghost as my primary eventually, though I do not like the remote as much as the Ghost.
My daily use setup right now is the Ghost S as a main POV helmet cam, and one or two Sessions for "B" camera, usually on a handlebar, mirror or swingarm. Works pretty well for me and is quick to rig.