We have stayed at both in the past year. We don't love the Newport Bay Club. It's a lovely hotel and looks great from the outside, but I find the set up of the entrance from Lake Buena Vista and the upstairs lobby a bit claustrophobic feeling (while you enter into a grand-ish staircase, the entry is actually quite small and flanked by restaurants on each side, which can lead to crowding). Like for like, the room sizes and appointments are similar. We also don't love the nautical theme which is a bit over the top and now feeling a bit dated. On the flip side, the Marvel hotel (as you know) is very recently renovated and everything is shiny and modern. We also like that the Marvel hotel offers three bars/lounges which we like to use to relax a bit out of room and out of park to grab a drink and/or snacks. The kids facilities are also nice and new and if you have kids into Marvel, that Marvel experience area down by the convention rooms is cool and good for some fun pics. All that said, I don't know that we feel the premium cost of the Marvel hotel is worth it compared to staying at one of the other hotels (and we're not massive Marvel fans, so while we love all the "updates" the theming itself isn't a big draw for us).
If I am honest, of the three Lake Buena Vista hotels, we prefer Sequoia Lodge. While it's technically a step down to 3 stars (not quite sure why), to us it feels more "cozy" than "dated" compared to the Newport Bay (and now that I think about it, the entrance is not totally dissimilar....but it's never bothered me at Sequoia Lodge). Rooms can be slightly smaller, but to us offer all the same amenities. We also really like the Redwood Bar and Lounge (and it helps that just nearby is a kids area where they can let off some steam if we're lingering in the lounge longer than they would like). It is a slightly longer walk to the parks given it's just opposite Disney Village on the other side of the lake.
Have a great trip. Where are you staying for Efteling? They've just opened their new hotel right at the park entrance and it looks gorgeous!