I have had the chance to play with both systems as I own both of them. The last few days I have been comparing the two HMDs in VR.
Tracking aside, because we all know inside out will never compare to Light House tracking, here are some of the major differences, pros, and cons between the two HMDs.
Despite the Cosmos having a larger screen resolution over the Index, the Index image quality is much better. Not only is the Image in the index sharper, I also prefer the colour reproduction in the Index over the Cosmos HMD.
This disparity was very noticeable in Dirt Rally 2 2 when I was playing. In Dirt Rally 2 the Cosmos has blurry edges and it looked like I was looking through a Vaseline smeared lens, the colours where also washed out.
DIrt Rally 2 in the Index was nice and crisp and the colours were crisp and natural. The detail was welcome in the Index, it allowed me to spot cars in the side mirror, I couldn't do that in the Cosmos.
The Dirt Rally comparison was kind of the worst case scenario. Other games the difference is less noticeable.
Note that for both HDMs I run the resolution @ 150% scale. Maybe the above is due to the lens differences between the two VR headsets.
I run the refresh rate on the Index @ 120hz, Cosmos @ 90Hz. For most games this is not a big deal, but there were a few instances where the fresh rate mattered, usually fast paced games. Overall I would say 90Hz is fine though.
The Index has a lot of God rays, I mean a ton lot. It's on par with the original HTC Vive, which I used to own as well. It's fine in a brightly lit setting but when there is a dark scene on the Index the God rays are very noticeable.
The screen door effect is less noticeable in the Index I know the panels have some sort of coating in the Index. Having said this the Screen door effect is not very noticeable in the Cosmos either.
IPD adjustment is way easier to dial on on the Index, it's more precise. However the IPD slider is not as secure on the Index as on the Cosmos. Violent and jerky motions, like in boxing games, I have experienced the IPD adjustment slide on the Index, not so much on the Cosmos.
Lens distance can be adjusted on the Index, not so much on the Cosmos. This is a nice feature, especially for immersion in VR. Cosmos lens positions is a bit further back vs the Index, this is probably to accommodate people with glasses.
On the Index I have my lenses as close to my face as I can without touching any part of my face.
Overall comfort, I prefer the Index, not a fan of the halo strap, the sweet spot is much harder to achieve on the Cosmos. I like how secure the Index feels on my face. In games where you have to crawl around the Cosmos feels like it's going to fall off my head, this is not the case for the Index.
This is a bitter sweet thought, because I do like the ability of flipping up the Cosmos HMD when I need to, but for more active games the Cosmos halo strap is a no go. I find that the Cosmos resting on your forehead allows for too much slippage and you loose your sweet spot too easily.
And loosing the sweet spot leads to more eye strain for me.
I find in order for the Cosmos to sit firmly on my face I really have to dial in the halo strap, this can become uncomfortable and cause pressure spots on your head. WIth the Index design I don't have to do this and can wear the HMD a little looser on the head due to the fundamental design difference between how you wear the two HMDs.
I have a big head and the Cosmos face plate/cushion has a better fit for my large round head. Also the nose flaps are better on the Cosmos, they let in a lot less light in between your nose and the HMD. These flaps are almost non existent on the Index.
Built in Audio is great on both HMDs. However with the Cosmos headphones I find it is harder to dial in the sweet spot, it's a very small spot. I am also experiencing an issue where one of the side headphones keeps popping up on me, this could be a defect on the Cosmos though, not sure about that.
With the Cosmos headphone you are immersed and you can not hear any of the outside world.
The Index headphones sit a bit away from your ears, the sound is great, and the sweet spot for the audio is quite large. I like the idea of hearing a little more of my surroundings with the Index and the distance provides a nice level of comfort and you don't sweat on the headphones.
The HMD cable is lighter and thinner on the Cosmos, you might not thing this is a big deal but it does make a difference during game play. A lighter and thinner cable is much easier to manage and handle during Vr sessions. The Index cable is thick and heavier, not saying it's hard to manage, but it does bunch up more, and handling it during game play is nto as easy as it is with the Cosmos.
Breakout cable connector seems a little flimsy on the Index. I feel like I'm going to break the connector on the HDM side every time I plug the index into it's breakout cable. The Cosmos does not give me this anxiety. The Index cable feels more secure in it's breakout cable connection, not sure yet if that is a good or a bad thing.
All in all both HMDs have their Pros and Cons.