Skating is the biggest difference.
Siemens is an elite skater with great mobility for a big guy. A great first few steps that can keep up with the fastest players in the WHL already as a 17 year old.
The two are pretty similar, but McIlrath is bigger and tougher and is definitely a more feared fighter.
Siemens is a great fighter though, don't get me wrong. I have seen him fight 20 year olds and handle them quite easily. People continue to underestimate his strength and fighting ability.
Here's a fight with Siemens from the weekend against Matt Fraser, who is a 20 year old and is 6'2 and almost 220 pounds. Siemens out muscles him pretty easily....
Along with that Siemens also has the better offensive upside. Both are solid passers who make a good first pass, but Siemens is much better at jumping into the rush and creating offense compared to McIlrath.
McIlrath is hands down the better defensive player though.