We have a product that runs now on Rift, Vive, and PSVR and will soon on Daydream and upcoming mobile and Windows VR. Take my opinion for what it's worth.
Comfortable VR requires low-latency motion sensing, screen displays with pixels on/off for precise periods to avoid blur or flickering, simulation and rendering that is at least 60 fps, and asynchronous reprojection of that output to 90 or 120 hz. All of the above VR systems are capable of comfortable VR running applications that meet that framerate requirement. Many would additionally argue that head-position tracking is a requirement for comfortable VR, because otherwise the world "moves" with your head. We say it's definitely better to have than not, as long as it has the same low-latency as rotation sensing and is reliable.
Applications also need to minimize the difference between acceleration you see with your eyes and feel with your balance. Our research shows people have different trigger thresholds for simulation sickness, and different sensitivities to different types of acceleration (for instance most people can handle differences in forward acceleration than vertical, and both better than turning). Different applications have many ways to address this: low detail backgrounds or background occlusion when turning, "cockpits" that turn with you, shuttering of FOV to reduce peripheral detail when turning, teleporting, acceleration limits, head-synced turning, level design that encourages more or less accelerations and vertigo, room-scale only movement, etc. You will have to jump in yourself and find what you are capable of and what applications do the trick for you.
The rest comes down to features and ecosystem of each VR system. Hardware systems have been evolving very rapidly but here's a brief rundown.
Vive & Rift are very similar from sensing and screen and computing requirements,wide fields of view, high application framerates, They both now require a tether to your PC with I5-4590 & GTX 970 or better performance. Vive came out with full room-scale position sensing and two hand-controllers, which has led to a lot of great room-scale applications. Rift came out with built-in headphones which are key to enjoying the full VR experience, as sounds can be "binaurally" mixed to sound like they are coming from precise locations, and is lighter than Vive. There are a lot of Vive add-ons available now or soon that include face covers, wireless transmission, tracking pucks and alternate head mounts with headphones.
PSVR actually has a higher screen refresh (120 hz) than Vive & Rift (90 hz) which makes looking around (with async reprojection) feel more crisp. But most PSVR applications run at 60 fps rather than 90 fps like most Vive & Rift apps, which makes object animations and positional travel less crisp. PS4 Pro apps can hit 90 fps at about the same level of detail but that depends on the developer. The PSVR's screen might be the brighest and uses a different pixel technology, less little dots and more solid squares, that is a matter of taste. It's a little heavier than Vive but is balanced between front and back so the weight rests on your forehead--in fact its screen guard doesn't even touch your face like Vive & Rift, and can move out and in for easier use by glasses wearers. PSVR's position tracking relies on visible light which is a bit less robust than the other two, though all of them have problems in direct sunlight.
Microsoft VR is further out but looks to be aiming for PSVR level performance on PCs with less than Rift/Vive specs. A notable feature is "inside-out" position tracking, coming from their Hololens research, which doesn't require external cameras like Vive/Rift/PSVR.
Daydream on a Pixel phone (Snapdragon 821) is surprisingly good for mobile. In our tests it has about twice the power of S7 running on GearVR, which our application can't yet run on with sufficient detail. Its applications require 60 fps but it has asynchronous reprojection to what feels like a 90 hz screen refresh. It doesn't have position tracking, and also has some gyro drift which can lead you facing a different direction the longer you play, and has a smaller field-of-view than Vive/Rift/PSVR. But I expect you'll see a lot more applications coming from them to Daydream, especially with its hand controller that can provide much of the same controls.
GearVR2 also has a hand controller, and a field-of-view in between Daydream and the Vive/Rift/PSVR. We're looking forward to it with the new S8 phones, because they'll have even faster chips (Snapdragon 835) that will close the performance gap even more. S7 phones don't hit the performance numbers that enable similar games to run on the other platforms, and the original GearVR lacked a hand controller, so I could not recommend them unless you are fine with much simpler games and experiences.
Battery power is obviously an issue that mobile VR has, expect somewhere around 10% for every 30 min of play. And apps that push their performance hard for a long time can be throttled to avoid overheating. But tether cords are obviously an issue that PC/console VR has, along with higher price and you can't take them places.
The last thing to consider is how much reuse you'll get from the computing side of the system. Rift/Vive require newish PCs, PSVR requires PS4, Daydream requires Pixel or newer Android phones that meet its specs, and GearVR requires S7 or other compatible Samsung phone. For most of our customers, the choice comes down to these which precede the cost of a VR system. All-in-one VR systems are also on the horizon, for less total cost but also no reusability outside of VR. One thing for sure is that VR now "works" in an increasingly consumer (as opposed to theme park) way, and is a pretty fantastic and unique medium that developers are now getting a handle on.