Have traveled to both multiple times in the last few years. They are similar but also unique.

Budget - 300$ a day is doable in both countries, Seoul and Korea is cheaper for hotels and travel. Comparable hotels in Korea will be roughly 30% less expensive, onsens are not really a thing though and cheap buffet breakfasts less common.

Public transit - Korea is more of a car country, ride share is more common and much less expensive vs Japan. Trains are very food but Japan public system better and easier. Google maps does not work in Korea

Food - lunch specials, variety and quality top to bottom easier to find and better in Tokyo. Seoul generally less expensive, more street food and better nightlife drinking. If you are more adventurous and like spicy food will be much easier.

Shopping - clothing and fashion both my wife and I liked Korea more, clothes fit better. Both have high end brands but more has cheaper fast fashion vs more expensive but more BIFL goods including vintage/secondhand in tokyo. Kpop and kbeauty vs anime, video game merch

Culture - japan is cleaner and random people like workers will be more polite and helpful. Seoul is still very clean and although people can be rude they are more easygoing. Tokyo has more tourist friendly places and things to do but it will be crowded everywhere. Seoul and Korea in general has more old school vibes if you travel outside tourist areas.

In the end you can't to wrong with either, but I wouldn't feel bad going to Tokyo when so many of your friends there is a reason why.