Japan is awesome and a must visit country. It's one of my favourite places to go in Asia and like Hong Kong it is Transformer heaven.
South Korea is fun but it isn't as tech and fast paced as Japan or Hong Kong. It is also one of the most regimented and consumer patriotic countries I've been to... try being the only people in an area with an iPhone because everyone has a Samsung, or only seeing Korean brand cars in one of 3 colours - white, grey or black. South Korea is also heavily influenced by the US so lots of US chain restaurants everywhere and streets full of beauty products which I find a little strange. I've never found any Transformers in Korea, although admittedly the last few trips we've been too busy to find any shopping malls.
On the plus side we've had some amazing ski trips there as it is a cheap place to ski (quality of slopes on par with Perisher but nowhere near as good as japan/US) and the people, while not super friendly are always curious (my daughter in particular draws a crowd for her alleged "perfect eyes" whatever that means). The local food is fabulous although if you ever go to Eastwood in Sydney you can find a pretty close match in terms of flavour and quality.
I like going to Korea but I find it a little harder to navigate than Japan in terms of language/culture. I love seeing all the fighter jets and stuff buzzing around. Bear in mind South Korea, like Vietnam, also had a major conflict as well and Seoul is rather close to the border with North Korea. They're technically still at war but have an "agreement" so it does have some risk associated with travelling there.
When it come to travelling, especially in Asia, I think you have to have an open mind.
I have been to countries I never would have imagined over the last few years and constantly been surprised by how different they are to how I envisaged.
I've been to places I didn't even think would be touristy - Taiwan comes to mind - and had an absolutely amazing experience.
What was that Aussie slogan... You'll never never know if you never never go!
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