I have a Kindle Paperwhite, my wife has the newer waterproof version.
I'll start with my biggest fear first, the "ad supported" is no big deal. When the tablet is idle an ad for a book is displayed on the screen. When you want to read a book you swipe to unlock the reader and you won't see another ad until it is locked and idle again. It's possible there are other ads that I haven't noticed in other menus or when buying books, but if they are there they are unobtrusive.
Looking at the feature list it looks like the extras with the Oasis, beyond having a 1" larger screen, aren't worth it to me for ~$155 more, but that is really my opinion. Since I've never put my hands on one it's probably unfair for me to make any kind of definitive recommendation. I'll give my opinion of the "extra features" with my experience using the Paperwhite.
- Larger screen size - I guess this is preference. For me the smaller reader fits in a jacket pocket if I'm bringing it somewhere where I may have to wait around like a doctor's visit or Jury Duty. I don't wear glasses, so I'm not sure if that extra inch combined with a larger font makes enough of a difference. I'm not knocking it, I just don't know.
- 25 LEDs vs. 5 LEDs - The PW seems plenty bright so I assume the extras are multicolor to allow for the warm light.
- Adjustable Warm Light - I may be interested in this as the PW is as described, a white light, but I've never had a problem.
- Auto-Adjusting Brightness - meh, tap the top of the screen, tap the gear, slide the brightness where I want it on the paperwhite. Also, sometimes I adjust the light brighter or less depending on other lighting beyond the ambient light, but I'm sure you can toggle off the auto adjust.
- Ergonomic design - I really don't know what this means, it is a rectangle. My only guess is the bezel on the right hand side instead of the top and bottom for holding. The would actually be a slight negative for me as I am left handed.
- Automatic rotating page orientation - I may be interested in this, but I'd have to see it in action.
- Page Turn buttons - Again, not sure. My OG Sony had them and I preferred that over Kindle when I first changed (I had an original Kindle before my PW) but I adapted fairly quickly.
Like I said, I don't see anything on the additional features the would make **
ME** pay the premium. Even reading further and seeing that it has an aluminum shell doesn't move the needle. Mine is from 2016 and I don't baby it and it still chugging along today. I've dropped it numerous times with no appreciable damage. I do have a case, but I think the larger factor is it weighs so little that when it does fall there just isn't a lot of mass.
The only update I would pay for is to get the waterproof version. Though I've only used it a few times on vacation it's nice to use in a pool. The larger reason is I always bring it camping and the likeliest way mine would get damaged is being left in a chair pocket or on a table in the rain.