I have only been to Iceland (jan 2023..first time out of the US and loved it's culture & many things about it) then in May 2023 visited Yellowstone. Iceland has a lot of natural wonders but Yellowstone has more varied natural wonders and all in a smaller area.
I'd personally recommend Yellowstone over Iceland if your looking to experience the best/most unique natural wonders (grand canyon, tons of waterfalls, a massive geyser Old Faithful compared to Stokkur, wildlife safari, hot and colorful out of this world scolding hot pools and more). I do need to do the ring road in Iceland, but Iceland surely does not have wildlife nor colorful hot pools (that i know of anyway).
I'd personally recommend Yellowstone over Iceland if your looking to experience the best/most unique natural wonders (grand canyon, tons of waterfalls, a massive geyser Old Faithful compared to Stokkur, wildlife safari, hot and colorful out of this world scolding hot pools and more). I do need to do the ring road in Iceland, but Iceland surely does not have wildlife nor colorful hot pools (that i know of anyway).