First off, those polysulfone heads aren't easy to cross thread at all, the big worry is snapping one of the in/out's off the main body. Ask me how I know
I just snapped my first one last week & called up Northern Brewer to ask about the Steelheads. They're made from a different manufacturer but I was told the heads are interchangeable & they're hoping to sell the head assembly ala carte in the next couple months.
If the little extra money isn't an issue, I'd recommend going with the steelhead. Otherwise the March's are great pumps as well. The heads aren't *that* easy to snap, but it's possible & will probably happen 5 years down the road... at the worst possible moment, of course.