My main reason for a plexi reservoir was to put lights under it and have the water do more than cool. I believe it is easier to fill and bleed the system too.
Due to the inline design of your res, I don't think you'd have to worry about a vortex forming. Just keep it full. I would also mount it on the suction side of the pump. In my first water rig, I soldered fittings on a horizontal mounted radiator for 1/8" pipe plugs on the radiator tanks opposite the inlet/outlet. I could remove the suction side while the pump was on to top it off without water surging out.
Now the placement of the fittings on my res invites a vortex if the res is not at least 7/8 full. Inlet is at the side, outelt at the bottom. I leave a slight airspace at the top so I get an everchanging light show from the air bubble moving around the top. At 3/4 full, I will get an attention getting air sucking vortex once it meanders over to the outlet and the pump gets a hold of it.