i've never seen a mid or top end step through, sloping frame is the strongest, step through only good if you keep the weight over the back wheel or if it weighs a ton, If you are investing the time, effort and money to build why start with an inferior frame. If you can find one a few manifacturers make curved top tube frames for just this reason, so you have the best of both worlds. I cant remember offhand who makes them but I know KHS does.