If before hand you are not really interested in manga and is really into anime, then try anime first
And if you are really into manga and not really into anime, well then try the manga first.
it fits according to what are you looking for, or what you are expecting.
If you can't decide. To me personally I'm someone who really appreciates anime, the OSTs, Moving and motion of charaters, voice acting, and the way its told in anime is something I really look forward seeing.
Then if, I get invested to it and find it something that I really like, that's where I try to transition into manga, especially JJK is still ongoing in its weekly release.
Basically my point is try to know what you are looking for or what do you prefer more personally, because this is gonna be your first time experiencing it.