I played not only both (on PC, 60 fps), but I immediately returned to Sekiro (and did a playthrough without Kuro's charm) immediately after beating Fallen Order on Grandmaster difficulty, so here's my comparison:
Obviously, Sekiro feels "smoother" to play. Don't expect the buttery smooth, polished, super-fast running, etc. in Jedi.
But, Jedi holds its own rather well, for a western game. It's pretty good at making you feel like a "Jedi", with all the powers at your disposal, both for combat and traversal/platforming.
There are some similarities to Sekiro for sure. There's a stamina system very similar to how posture works in Sekiro. Whittle down someone's "guard" (or stamina or call it what you will), and you can one-shot most enemies. Cal doesn't require stamina to run, attack or dodge, but does lose stamina of his own when blocking, so you can get guard-broken as well. There are meditation points that act as bonfires (respawn enemies, refill your heals, acquire skills, etc.).
For difficulty, well, I played on Grandmaster, then briefly lowered it to Jedi Master, then was bored and put it back on Grandmaster. For Sekiro players I recommend Grandmaster. It's very, very difficult
at first; Cal will get two-shot by most everything, some optional mini-bosses early on will even OHKO him. But as you progress a bit more, you gain more skills, more HP upgrades, more healing stims, etc. and you can survive better. I regretted lowering it; I played most of Kasshyyyk on Jedi Master and I pretty much never died and killed the boss in one try, whereas on Grandmaster I'd die pretty frequently and needed to constantly be on my toes.
Compared to Sekiro, Grandmaster is close, I think. Early game on Grandmaster might be harder than early Sekiro, but later Sekiro is much harder than later Fallen Order. While the last two bosses took me a few tries, I certainly didn't retry them as often as I did the final bosses of Sekiro. So overall it's probably easier, but early game will be a bit brutal. So, it's not as well balanced as Sekiro's default setting, but it's still the most fun one for me and the later bosses were quite satisfying to beat.
When you progress into the game, Jedi gives you a lot of tools to handle encounters that would otherwise be difficult in pure combat. For example, you get a lightsaber upgrade that makes mass-deflecting blaster spam very easy and nukes everyone in sight. You can mass Force Push a whole bunch of people, including "elites", off a cliff. Some elites that are otherwise not trivial to melee, can be Force Pulled into your lightsaber for a OHKO. You also get lightsaber upgrades that do massive damage and/or have very good crowd control. Etc.
People who insist the hardest difficulty setting is easy are just flaunting assholes.
No one did that though? Jedi Master is not the hardest, but it
is very easy for experienced Souls/Sekiro players. Grandmaster is the hardest, and is quite challenging.
Yup. There's a reason "put it on easy and enjoy the ride" is such an overwhelmingly common piece of advice for this game.
Not for someone who enjoys games like Souls or Sekiro.
I got mildly frustrated at some point and lowered it to Jedi Master and then I found the game way too easy to the point of losing its excitement, so I put it back on Grandmaster and I'm glad I did because the final few bosses were a lot of fun and required me to use all my Jedi tricks and powers to win.
Taron Malicos, a late-game boss, was the most "Souls-like" boss and was a lot of fun to fight.
