I recently had two new wheel sets built. One set with dt 240's, cx-rays and deep-v's and the other with dt 240's, 14g dt's and pro elites(tubular version of the deep v's).
With what you want, I would go dt 240's, dt spokes 32/32, and the niobium(assuming them make them in 32 hole).
While they might be a little heavier with the extra spokes and a little less aero I think they will hold up better and the difference won't be that much. For comparison, I went from dt 240's, dt butted spokes and Mavic Reflex tubulars to my current wheels. There is a slight difference in the responsiveness, but I feel like I put the power down much better now, the bike definitely rides better and I just like these wheel sets infinitely more.