The US government classifies both of them as languages which are very difficult to learn, based on how many hours it takes a native English speaker to gain fluency. How does Hangul stand up against Hiragana and Kanji? I'm going to cover some of that in this post.
Since I have been studying them both for a time I can say this out of experience. If you start with Japanese later on Korean will be more easy and I think it's the same vice versa. I will introduce both of the writing systems to you.
Hangul consists of 14 consonants and 10 vowels which are all combined in syllable blocks. As an example 한글 (hangul) is divided into two syllable blocks. The first one is divided into three letters; ㅎ 'h' + ㅏ 'a' + ㄴ 'n' which spell 한 'han'. The second block 글 'gul' also consists of three letters; ㄱ 'g' + ㅡ 'u' + ㄹ 'l'.
I can say for sure Korean is more difficult in terms of pronunciation. For example the use of 'ㅂandㄱ.
'ㄱ' or 'ㅋ' is pronounced as 'ㅇ'(ng), when the latter word is started by 'ㄴ,ㄹ,ㅁ'
'ㄱ' is pronounced as 'ㅋ'(K) when the latter started by 'ㅎ',
'ㅂ' is pronounced as 'ㅁ'(m), when the latter started by 'ㅇ', and the latter is a compound word.
'ㅂ' or 'ㅍ' is pronounced as 'ㅁ'(m), when the latter word is started by 'ㄴ,ㄹ,ㅁ'
'ㅂ' is pronounced as 'ㅍ'(p), when the latter started by 'ㅎ
But, you only have to memorize 24 characters. How great does that sound compared to the Japanese writing system which has three alphabets!
The Japanese writing system can be divided into three 'alphabets'
When I studied Japanese I always explained the Japanese writing system simply like this.
Kanji is based on the Chinese Hanzi characters and is used for nouns. This means every Kanji character has a own meaning and even multiple meanings sometimes. Katakana is used for foreign words adapted in Japanese like ラジオ which is the Japanese word for radio and is pronounced as 'rajio'. Hiragana is used for everything else, for example particles like の.
These three alphabets can be combined in one sentence. I will give you an example again;
私のラジオ = (watashi no rajio) = My radio
Kanji = 私 (watashi) = I
Hiragana = の (no) = states possesion
Katakana = ラジオ(rajio) = radio
As you can see I used all three alphabets in one sentence. Also 私 means ' I ' but together with の it becomes ' my ' . I think this is what makes the Japanese writing system beautiful but also shows how difficult it can be.
There are a lot of particles like の and if you don't understand the particles, making sentences can be really difficult.
In comparison to Korean particles I think Japanese has the advantage in easiness. While in Japanese the particle ' は ' is only written as this, in Korean you have to choose between ' 은 ' or ' 는' . For the character ' を ' it can be either ' 을 ' or ' 를'
I think they are both equal in terms of difficulty. The downside on Korean is mostly pronunciation and the difficulty of the particles. The upside is you do not have to learn any Chinese characters. Many people say you can master Hangul in a week, I only agree to this concerning the writing part for the pronunciation you need a lot more time.
Then the downside to Japanese is of course learning all the Kanji. Again I hear a lot of people who say the Japanese pronunciation is really easy. I also read this everywhere and I really have to disagree. One of the things I thought was really important while learning Japanese was perfecting my pronunciation. I have seen so many people who studied Japanese for many years and even if you blindfolded me I could hear their pronunciation was not good.
You should not take both of them lightly but it's very rewarding speaking either one of these languages.
I wish you a lot of luck.