I'm thinking about my own experience and I've gone both ways. On my first bike (in 40 years) I pretty quickly replaced the saddle but rode the original tires until they wore out. On my current bike I replaced the tires after I'd had the bike about a month, but am still riding with the saddle it came with. My inclination (with little information) is to say replace the saddle first, but that's not what I did on my current bike, because I'm happy with the saddle it came with but wasn't happy with the tires.