I was Senior before interviews, I was mainly targeting Staff level when I was interviewing. No convincing needed, I had enough experience to get to Staff level. With Startup I asked them for level up but they said they had limited number of positions and all they can do is Senior at a higher end of it, and can't do Staff.

I worked at the startup for a few months before the other offer was finalized. So I have a chance to get to know what it feels to work at the startup as well.