Rishiri Island vs. Rebun Island?

Been to Japan over 25 times but never to the Rishiri or Rebun Islands. I'll be returning this summer but am slightly time constrained. If you had to choose between Rebun or Rishiri Island which would you choose and why? (2 or 3night stay)

Unless you want to climb Mount Rishiri I recommend Rebun because it has - in my opinion - a more varied, interesting landscape and the better walkign trails.

Then again, if I had the luxury of 3 nights, I would visit both.

With three nights I would stay two on rebun and one on rishiri. You can rent a scooter to explore but make sure to reserve it beforehand.

If you had to choose between Rebun or Rishiri Island
I have visited Rebun twice and Rishiri once. My recommedation would be obvious, go for Rebun.

Thanks for all the helpful replies. If we stayed one (or both islands) would it be best to stay near the port area? I'm hoping to arrange a hire car? (But from past experiences at other remote islands I know a rental car is not always guaranteed)

Have you checked the accommodation?

June and July are the most crowded months for these two islands. I checked the most popular inns in Rishiri (Rera Mosir)

http://www.maruzen.com/tic/oyado/

and in Rebun (Pension Uni)
https://pensionuni.main.jp/

and they are almost booked out. Of course there are some other, but not so many I think.

We went there in 2020 August for 7 nights (3 in Rishiri and4 in Rebun). August is less crowded because of the less flower. Rebun is known for trekking (there are several trekking routes where you can’t go through with car) while Rishiri is for climbing. We didn’t climb so we rented a cheapest K-car for 4 days in Rishiri and go around the islands every day. In Rebun Pension Uni provides shuttle service to the port where you can ride the bus to the starting points to the trekking routes. I believe most of the accommodations in Rishiri and Rebun provide shuttle service to the port and the airport (only in Rishiri).

For Rebun, you might find this map helpful:
https://www.town.rebun.hokkaido.jp/hotnews/files/00000100/00000181/reb...

You might want to consider tour buses in both islands:
http://www.soyabus.co.jp/app-wp/wp-content/themes/Souyabus-child/pdf/c...