Which is the better running back for fantasy this year? Portis or Slaton '?

I have not much depth at WR and someone in my league has not much deptch at RB. Since we're opposite, i thought id trade eitehr portis or slaton for fitzgerald or calvin johnson. should i trade one of them? if so which one to get which WR? or should i keep them both? please help and thanks agaiin.