The Venturi would have lower losses, because the energy loss is small. Typical discharge coefficients (C) in the .984 range. In essence, this means 97% (.984 squared)of the energy is recovered between the inlet and the throat of the venturi.
So, there's surely a little more loss in the downstream section of the venturi. Maybe it's just as much as on the upstream. So, let's say another 2%. Loss is say 5% or so tops if your venturi surfaces are not craggy.
To determine the losses in an orifice plate, check the ASME Standard MFC-3M. The C value for an orifice plate is 0.5959+0.0312xB^2.1-0.1840xB^8+91.71xB^2.5xRD^(-.75) where B is the diameter ratio d/D, and RD is the reynold's number of the pipe. This is, I'm nearly certain, empirical.
Since C is proportional to the flow, and the energy loss is proportional to the square of the flow, the ratio of the C's squared should be the percentage you want.
But in rough terms, let's say the venturi is almost lossless - that is, the pressure difference between the inlet and the troat of the ventri is recovered in the downstream section. If you measured the pressure difference from inlet to outlet it would be quite small. The orifice, the water horsepower loss would be rho x g x deltaH where rho is the density of the fluid, g is the gravitational constant, and H is the head in feet. In English units, that's Q*deltaH/(3960*sp. grav), where Q is in gpm and delta H is in feet.
This is probably more than you wished to know. Please check my math.
