First off, Newbury Park runs more like 70 miles per week. I watched three Newbury Park runners on Strava who ran 70 miles per week religiously, despite what they say.
It's not necessarily the miles they run, but they run VERY hard workouts. So, while it doesn't mean they can't improve quite a bit, there is a greater chance they're closer to their ceiling than someone like Gary Martin who's training is not as intense at this point in his career. There are a million variables, but from that perspective, Martin has more upside.
There are many cases of hard training high schoolers who have improved in college (see Ritz, Solinsky, Nico and many others), but there are also many cases of those who trained very hard who didn't pan out.
While Nico has improved nicely in college, he absolutely dominated in high school and several from his graduating class have significantly closed the gap on him since high school and it's not out of the question that several will pass him in the next year or two.
It's impossible to know who will be better in the future, but from strictly who is performing better now while training less, Martin is clearly a safer pick to improve more in college.