Depends on you, depends on lifestyle, depends on baby.
DC1 I thought I’d have both and prob use 50/50. I loved twirling that bloody pram around John Lewis before I bought it.
He had other ideas and hated being put down, and I loved carrying him. Realistically I did not need the buggy for the first year, and even after that it was always more sling than buggy.
DC2, I thought I’d learned my lesson and only intended to have a sling, in the early months at least. When she was a few weeks old I ordered a buggy after all 🙄 because it turned out she quite liked napping in the Moses basket (as well as in my arms), and it was really useful to be able to stick all the stuff for two children under the buggy rather than carrying it. It was a super hot summer and nice not to have her literally stuck to me. She’s nearly two and I think overall she’s probably been about 40% sling, 60% buggy.
Personally I think the one I absolutely couldn’t have been without was the sling, but that’s based on my lifestyle and my sample size of two!