Callum Smith became very predictable and overly relied on his left hook to the body and head against Beterbiev. When you fall into patterns you become very predictable and easy to figure out. Plant has more variety but he fades later on.
I think Smith is better, has more power. And I think he'd win that fight. The Canelo fight looked bad but he moved up straight after meaning he was struggling to make weight. Plant fought Canelo at a comfortable weight. I highly doubt Plant would last that long against Beterbiev. Smith and Plant would make an interesting fight, anyway. But I don't see Plant hurting him and I think Smith has more power than Canelo so could KO Plant.
Both overrated when they were on the way up. Neither's skills are anywhere near elite. I'd say Plant has the slightly more rounded technical skill set which is why he did better vs Canelo. But Smith has size and power on his side and if they were to actually fight each other I think Smith much like Benavidez would get to him too often due to Plant's lack of counter punching giving Smith nothing to fear when walking him down. So Smith probably beats him due to styles.
Smith vs Plant would be a good one but like @Brighton bomber, I'd tend to favour Smith due to his extra power and size. I don't see a Plant win as out of the question though, he does have skill and I could see him being elusive enough that Smith doesn't get to him.
Plant is a "better" boxer, who's a better fighter? Callum is physically stronger and more durable..................and I'd pick the guy that can beat you up and take incoming for 12 rounds over the guy that is gonna start falling apart after 8 rounds..........
Smith got walked down and should have lost to Ryder. Plant would shut out Ryder. Smith is not even in Calebs class either and would get his ears boxed off. If Mungia destroying Ryder didn't convince you then I don't understand.