I have a Sebastian so am biased but I do know a lot of Harry’s, it’s very trendy. Love both though.
My Sebastian is called Seb most of the time, or Sebbie... I love love love Seb and rarely use his full name, but it’s nice to have the option and Seb and Sebastian sound very different. I do think Sebastian is quite formal and we do get posh comments, bur Seb is just lovely.