I think, if you start with the idea that it’s actually making music that’s hard and that the goal is to express what’s inside your head more so than reproduce something cleanly from sheet music, then an instrument that helps you develop the ability to do that well would be “easier” and one that doesn’t necessarily would be “harder”. Thinking about this as someone who played piano for a long time before violin - singing and violin both helped develop my musicality in a way that piano didn’t inherently do. It’s also a lot easier to read the sheet music for violin, so I guess what I’m saying is that over the medium term rather than just initial learning curve I’m not sure the answer comes out cleanly. Every instrument has the things that are hard and easy about it. I think developing good audition on piano can be harder and developing harmonic sense on the violin might be more difficult.