I've seen very little of Nurkic on the court, which isn't surprising, since he's injured so often. Comparing him statistically to Ayton, he's clearly a step down, but it might not be too bad if he can stay healthy.
He's more versatile offensively, having adding a three-point shot last season (.361 on 2+ attempts per game) and being a better passer (3.4 assists per 36 vs. 1.9 for Ayton, an a slightly better A/TO ratio).
Also, according to the statistics, he's not much worse defensively: a defensive rating of 114 last season compared to Ayton's 111. Over his career, Nurkic is a better shot blocker, but his numbers in that department have dropped off significantly since whatever his big 2020 injury was.
I'm disappointed in Nurkic's bad free throw shooting (.671 career, with no upward trend line). His foul rate is much higher than Ayton's (4.7 per 36 vs. 3.3). Even more than his health, Nurkic's inability to stay out of foul trouble means that he can't ever really be more than he is. Ayton presumably still has upside, although many of us here have our doubts.
It's maybe worth noting that Nurkic's primary backup last year was ... Drew Eubanks. Maybe they have a good rapport.
Nurkic is never on the short list of the NBA's impact centers, but, if we're being honest, the only reason that Ayton is in the discussion at all is because he's a still-young former #1 overall pick and the Suns are a marquee franchise. Put Ayton on a mediocre team, and he'd be no more relevant than Nurkic is.
I don't know Caleb Martin's game either, but from the stats he looks like your standard 3-and-D hustle guy. He rebounds pretty well for his size.