Amsterdam & London or Bucharest & Istanbul?

I’m an 18 year old male going interrailing in a month from Finland by starting in Lyon to see family and then going through Switzerland, Italy, and ending up in Budapest after which I still want to visit / have time for two destinations.

I’ve narrowed down the routes to the following two:

* Budapest -> Amsterdam -> London
* Budapest -> Bucharest -> Istanbul

I would be taking a night train for every leg besides AMS -> LON. The first city I would have 3 days to spend in and 4 days in the second.

I’ve never been to Amsterdam but have been to London. However I would love to visit again to check out universities and enjoy the city solo for the first time. This is the more expensive option but there’s a more clear cut range of activites and sights I’d like to do.

Bucharest and Istanbul are completely new to me. I’m not sure if there’s much to see in Bucharest but it seems like a fun place to check out to bridge the gap to Istanbul, which seems like a fascinating culture shock of a destination for someone who’s never left the northern half of Europe. This option would be cheaper but also less safe for a naive first timer like me.

I’ll appreciate any and all input! Thanks in advance.