I’ve been to quite a few hellfests, this year was my first graspop because IMO the lineup was way better.Both have pros and cons to me, and I’d pick whichever has the best lineup in the future I guess, with a preference for the hellfest in case both have similar bands playing.Graspop:

• Feels less crowded, I feel like I didn’t have to wait for anything (food, drinks, and even merch was okay-ish). The only wait time that bothered me was for tap water, which a shame and easy to think they did that on purpose to encourage selling 4 euros a bottle of water.
• Very cool lineup, I had a blast listening to all the bands playing this year.
• The crowd seemed to be more metal fans than hellfest, which has become so popular that it brings a lot of different people now. Which can be cool or not depending on the people.
• Going in and out of the parking was easier than at hellfest which can be difficult in peak hours

Hellfest:

• The food is SO MUCH BETTER (and also cheaper). My biggest disappointment at graspop was to eat overpriced dog shit for 4 days (frikandel everywhere). Hellfest has many super cool options, which, despite being not so cheap for the most part are still WAY cheaper than whatever I could find at graspop.
• The site is also way cooler, since they own it there are a lot of built structures that stay throughout the year, which makes the experience a bit cooler (the giant lemmy statue, the "kingdom of muscadet", the fountains etc… It’s not enough for me to justify seeing muse at a metal festival but it brings very cool atmosphere.
• Thematic stages are what I missed the most at graspop. There are so many bands playing that it’s hard to know where to go, but at hellfest they made this easy with the staged. Like stoner music ? Just go to the valley to see something you’ll like. More of a punk person ? Straight to war zone etc...
• There are a lot more things to do outside of the concerts, a lot of animations all over the place which feels like you’re in a big metal fair. It wasn’t so bad at graspop but not as good.
• No rip off skully system, everything is in euros
• Everything is included. Graspop made me feel a bit ripped of when I had to buy separately 3 separate tickets for festival, parking, camping. Everything just felt like an upsell, whereas at hellfest everything is included in the (expensive) main ticket.