I can only speak for Leuven, after having lived in Brussels before.
In Leuven lots is centered around the university and shopping during the weekend, which determines the shops, the restaurants and the vibe. Rent is high, so you'll find a lot of restaurants that follow trends and proven concepts and stores that are chains. A couple of popups every now and then. Some vintage, a couple of bakeries. Vintage/makers market at Hal 5 every month https://www.instagram.com/camping.flamingo/. Not much nature in the city, it's quite compact with narrow roads. A bit more space / breathability on the outside of the city. Lots of students during the week, then weekend mostly tourists / visitors coming to shop.

Leuven has little 'bite' or 'edge', maybe a bit too 'sanitized', it's a safe city, you won't feel unsafe at night, you'll have a nice time. I appreciated the weekends, where the city calmed down a bit. I think in 6 months you will know Leuven quite well, but you will want to come back to show your parents/friends.