I don't think of them as capable of being compared.
There's the real vs. the ideal. In this sense, pragmatism is real, practical, down-to-earth. Let's do something that's going to work.
Radicalism, however, is not necessarily at the other end of that spectrum. That is, radicals do not necessarily have their heads in the clouds. Communism is idealistic, but Marx taught that the end justifies the means - a pragmatic approach to a radical goal.
People often equate "radical" with "extreme." A radical cure would be one which most people would consider impossible, or dangerous, or rediculous, or unheard-of. (But it might work.)
In the 50's they taught us that in the political spectrum we have moderates in the middle, liberals on the left and conservatives on the right - and then we have the extremely conservative "reactionaries," and the extremely liberal "radicals."
Many considered Bill Clinton pragmatic. Whom would we call "radical"? I dunno. Maybe Fidel Castro?