gmod and sfm both specialize in two different avenues
i've always said that SFM should be used for more up-close, detail-focused, "smaller" poses than gmod, and not for large poses with scenebuilds due to sfm's memory problem, whilst gmod should be used for those large poses with large amounts of props and models because it can actually handle it
it's not often you can see a person bridge the gap between gmod and sfm but it is possible however
and in the end it comes down to your personal preference and posing style
only inexperienced or stubborn retards will tell you to only use one of the two and to not use/try the other