Here goes my long winded answer to this question, which I suspect is on a lot of people's minds.
I have the Vive, tried out the Rift and have Gear VR.
This is based on my personal experience, so it's anecdotal evidence.
First of all, if you can, I would recommend heading to the nearest Microsoft Store to try out the Vive IRL.
Vive and Oculus viewing quality is pretty much identical, so you can get a rough idea of how each one would be like, by testing one out IRL.
"Immersion" and the "quality" is better on all HMDs (Rift and Vive).
Given the fact that the average JAV VR quality isn't quite their yet with low bitrates and low framerates, you're not losing much by watching it on Gear VR.
However when the source video file is optimized for HMDs, it is a different story.
The resolutions that can be displayed by HMDs and Gear VR with S7 and higher are pretty much the same, but because of the better lens on the HMDs, better fit as well as better options to optimize your viewing experience, viewing it on HMDs is more "immersive" and better in "quality".
I found that you can still run Rift/Vive VR videos on Galaxy S7 and up without much problems.
HMD VR files have higher bitrates, but the resolution tends to stay about the same with Gear VR files.
When I tried them both on the Gear VR, I noticed some differences, not significant, but there.
Your experience may vary, since some people claim there is a noticeable difference whereas some people claim there isn't any difference between a Gear VR source file and a HMD VR source file being played on Gear VR.
If you're int he market for the Gear VR, I recommend the 2016+ black/darkblue Gear VR.
It provides a better viewing experience than the older white one since they changed the lens for better FOV and a "clearer" viewing experience.
Word of caution,
Anything below the S7 may run into performance issues when trying view HMD VR videos.
For example, I tried to run Rift/Vive Porn source file on a S6 and a Note 5.
On some files, both phones heated up noticeably, dropped frames and sometimes stuttered, and the battery drain is a given.
In conclusion, going by a given that all of us are on a limited budget,
At the current stage I don't see a significant reason for someone to get a HMD over Gear VR, if your only purpose is to view VR JAV.
If you have hundred of dollars in disposable income to spare, then sure, go right ahead and enjoy!
If you have other needs for it, for developing, gaming, etc., go right ahead and enjoy!
If not, you're better off waiting for JAV VR to improve before you jump on the HMD wagon.
Even Porn VR has yet to use the full potential of VR on HMDs.
So when JAV VR uses the full potential of HMDs, that's when I can whole-heartedly recommend spending your hard earned cash on HMDs for Porn or JAV.
Lastly, one bit of news that should be taken into consideration is the recent introduction of Microsoft Mixed Reality Headsets.
Don't let the name fool you, these are mid ranged VR headsets.
Acer and HP are currently offering developer editions, and they seem promising for VR Porn and VR JAV purposes.
It's somewhat of a mixed bag.
On the plus side, their displays have higher resolutions of 1440p and the design is leaner and comfortable compared to current offerings.
On the down side, the displays may be LCD and not OLED, while FOV is lower.
Ultimately, most crucial, their price tags are much more forgiving at around $299.
For just Porn viewing purposes it may be a great alternative when the consumer versions come out.
Hopefully people with the developer editions can comment on VR Porn on reddit or somewhere so we can know how well it does.
If not, you guys can crowdfund me one and I'll compare it to the Vive and the Gear VR
