The 6" wheel will make the bike a little wider and the tire a little shorter and fatter, where the 5" wheel will keep the tire a bit taller and your track width closer to stock.
I have several 21-7-10 on both 5's and 6's, I need the 5's on some bikes just so they will fit in my toyhauler against other bikes.
Both will work, matter of personal choice.
I'm not sold on "bumpsteer" being the proper term, although very commonly used, but you will want to make sure your steering stops are up to snuff with a wider offset wheel.