I haven't played Hollow Knight, but Shovel Knight is awesome.
I don't know how long Hollow Knight is, but depending how good you are Shovel Knight isn't very long, you can pretty easily do it in one longish sitting. If you want to get one out of the way, start with Shovel Knight. Otherwise play Hollow Knight and let me know if I should buy it.
From what I have been seeing on the internet people are nuts about Hollow Knight. And it won best MV on a bunch of website I looked at. Every YouTube review I watched said it was amazing. I also ened up buying Dead Cells to because people are saying it's really good as well.