Jacsc,
Considering the size of piping you probably have for Steam Trap Manifolds it would not make a big difference between using a Full Port or a Reduced Port Ball Valve.
Count up the number of Steam Trap Manifolds you will likely have on the job, multiplied times the number of valves at each manifold times the cost for (a) full Port Valves and then (b) Reduced Port Valves.
I would choose the less expensive option.
prognosis: Lead or Lag