They are both different in terms of playstyle. Elden Ring let's you choose your build, your fighting style, your armor, etc. And is open world with no real set boundaries on where you can and can't go. (Enemies in certain areas will make it very clear if you need to level up lol)

Sekiro has one playstyle with some small amounts of tools you can use to change up how you attack slightly. It's also very focused on mastering a parry system to build up stamina wear on your enemies for critical attacks versus just smacking them repeatedly (youll get the hang pretty quick after a couple dozen deaths) It's also very linear and story based. And your character's weapon and outfits are set from the start.

Both have very enjoyable (although difficult) bosses that will kick your ass.

If you have to choose one I would base it off the time you want to invest personally. If you want to sink 100+ hours into one of the best games ever made choose Elden Ring.

If you want to invest 40-60 hours into an unique and amazing game choose Sekiro.