I have come from the note 20 ultra. Honestly I really like the s21 ultra. I like both designs but using curved corners after a year and half feels like a change. Having the larger s pen feels more comfortable in the hand. My tab s7+ s pen feels better then the s21 ultra s pen due to it being an actual pen size. I still like the notes but unfortunately my note 20 ultra had a terrible green tint issue. So I kept the s21.
Also the S21 Ultra doesn't come with an Spen. You can ideally use any Spen from the tab series or note series. Apparently the current one doesn't support the Bluetooth functionality. There will be one coming which will Apparently have the Bluetooth functionality sometime this year.