Comparing Rabble in Arms to Arundel misses the point. They are, for all their shared themes and characters, different books. Simply put, Peter Merrill's voice and perspective in this novel differ in many respects from those of Steven Nason in Arundel. Cap Huff, who returns as the errant knight, is integral: he is to Roberts as Falstaff is to Shakepeare. I do recommend reading Arundel first, since you will derive extra benefit from the character development that has already taken place in the first installment of this series.
Rabble in Arms is another great work by a very talented and prolific author. Perhaps Roberts' sympathetic treatment of Benedict Arnold and his ambivalence concerning patriotism and loyalty have kept his work out of the classroom. Whatever the reasons, it is tragic that Roberts and his novels are not better known. They deserve to be widely read.