Which city to combine with Taiwan? Seoul or Hong Kong?

China visa can be harder. I love Hong Kong but it's becoming more modern and less exotic, under direct China rule. Korea is more work in English than is Hong Kong, and you might find Busan better for hiking, but I've only been to Busan and Seoul. Korea is a huge country.

If you're looking for hiking, Hong Kong has good and accessible options. But I love Seoul - very friendly, great food and interesting cultural sites, so that would be my suggestion.

There’s a 144 hour transit visa that is easy to get to visit China, but I think HK has a different visa process since it’s technically run like a country. Heard no visa needed for US citizens!

If you like slick, modern cities, Seoul has a lot of that going for it. Also consider one not yet mentioned: Singapore.

You can't go wrong with either one but I think the weather will be a little better in Korea for outdoor activities that time of year. Hong Kong can start getting hot and humid by the end of March - rainier too.
A multi-city itinerary, flying into Taipei and home from Incheon, would be your best bet I think. There are a number of discount airlines that will get you from Taipei to Seoul pretty cheaply.
Might add that there are two airports in Seoul - the large international airport at Incheon and an older city airport called Gimpo which is located on the outskirts of the city versus the longer (about an hour and a half) trip out to Incheon. Might be worth researching flights into Gimpo as well.

Hong Kong doesn't require a visa. If you have never been to HK, you should visit sooner rather than later before China completely ruins the place over the next decade. HK has a reputation for being a big city, but the territory does have a lot of rural areas, and smaller Islands where there is very good hiking to be had.

Former expat. In the current environment, I will not return to Hong Kong. Very sad. That's a personal decision - your interests are well covered in HK.

Hiking - Dragon's Back is moderate at the hardest with great views. The Peak has lots of areas to hike, none hard. Lion Rock. Lamma Island - ferry to the Island and walk across - quite easy paved path. None of these trailheads are hard to access by public transport. There are other trails that I have taken taxis to - you can google trails and if you are tempted by a trail further out, you can probably still get there by taxi. (You may need someone to write the address of the trailhead in Chinese characters for the driver.)

HK museums - lots, some quite specific interests. Some bigger ones with easy access via public transport- Hong Kong History Museum, Hong Kong Heritage Museum, Hong Kong Museum of Art. There are others that are also quite good. Consider also Po Lin Monastery (Big Buddha), Wong Tai Sin Temple, Chi Lin Nunnery and a number of other temples in various areas of the city.

I never found the weather unpleasantly humid in March, but there can be days of rain with little let up. English is an official language. No problem with signs, major tourist sites English is well spoken; smaller restaurants, taxis may not speak much.

Seoul - hiking at all levels in Bukhansan National Park. There are multiple entrances to the park, but several are accessible by the subway system, or by bus. If you google, there are lots of blogs about different trails and access. (I visited and hiked in November 2023.)

Museums - a number of interesting ones. The National Museum is very, very fine, especially if interested in archeology. War Memorial (and museum). National Folk Museum. several art museums and specialized museums. The Palaces - especially Gyeongbokgung and/or Changgyeonggung. Between those two, there is a traditional neighborhood - Bukchon Hanok. Easy walking in the area and quite fascinating. A day trip by public transport, Korean Folk Village.

Signs are in English. Google maps doesn't work - you need to use something like Naver or Kakao. English spoken at major tourist venues, sometimes limited but people are very helpful. Younger people are more conversant in English.

Neither choice is wrong!!!

One area where it’s hard to make a choice is the season. It is cherry blossom season so I’m considering Korea for that reason, but I also saw them when I went to Japan!

And Hong Kong a lot say to visit ASAP due to future country changes, so it’s a hard decision!

Might need to just watch videos of the attractions people here listed

You are truly spoiled for choice between the two!

Korean culture is different from Chinese culture (Taiwan or HK) if you were interested in more of a contrast in that aspect.

So many great options! Since you've seen cherry blossoms in Japan, I'd lean towards Hong Kong just for the variety in culture and sights. But really either one would be amazing. Watch some videos and see what vibes with your interests most. You can't go wrong - have an awesome trip!

South Korea is admitting residents of the US through December 31, 2024 (for stays of 90 days or fewer, others will still need it), without the visa usually necessary. Might be the time to take advantage of this.

Have you considered Hanoi? It is a fascinating and charming city. There are alot of opportunities outside of the city to enjoy hiking and nature.

If you are considering Hanoi, I think you would like a day trip to Tam Coc, the area is also referred to as Ninh Binh. Vietnam is inexpensive and there are day tours that pick you up early at your hotel and bring you back. Biking and hiking heaven.