I’ve been to both a few times. I don’t think Portugal is the most suited for hiking and nature specifically, though I’m sure there will be nice areas where you have great hikes and nature. Wasn’t too impressed with the nature in Algarve and around Lisbon/Obidos. There is one absolute Portuguese gem though, imo, which is the island Madeira. It is extremely pretty, life is very laid back, been there twice and completely in love with it. The hikes go from very easy to very tricky.

We are in love with Croatia, too, so my recommendation is definitely Croatia. The small towns are gorgeous, food is excellent, great wines, beautiful islands, we cannot get enough of it. Prices are rising, especially Dubrovnik is ridiculous. But if you go in shoulder season, it is still quite possible to avoid the biggest crowds. Plitvice can be visited very early morning before tour busses arrive. Plitvice is the most beautiful thing in Europe imo. There are off the beaten track places too, like Lastovo island.

Bosnia is indeed also beautiful, and very cheap and not so overrun by tourists (except Mostar during the day). Easy to combine with Croatia if you have the time and quite a different experience. Would go back in a heartbeat.

Slovenia is also a gem for nature lovers. I am not a big fan of Bled (most popular tourist attraction), but the Triglav area, Soca valley, and the lesser known Logarska Dolina and Jezersko valley are stunning.

I’ve been to both too and my vote is for Croatia. Like others have already said, it checks off all your boxes. I also wouldn’t rush back to Portugal but I sure would go a second time to Croatia!