You can use either, however, the closed impeller may have a better hydraulic efficiency, especially in the smaller pumps units.
What is your hydraulic duty and installation configuration, this could influence the pump selection.
I will assume that you are speaking of an impeller in the 4500 Ns and below range. Typically, impellers of this type are referred to as enclosed or semi-open. The semi-open impeller eliminates the lower shroud. In this case, the suction front head, or diffuser body becomes a hydrualic surface. Because there is no lower shroud, there can be no wear rings on this type of pump.
Semi-open impellers typically exhibit HIGHER efficiencies. For a 1500 GPM vertical diffuser (commonsly referred to as a vertical turbine) pump for instance, a semi-open impeller may be as much as 2% more efficient. This efficiency is in some regards a false efficiency however. This is because the axial setting of the pump is critical. As the vane tips wear, efficiency will drop off dramatically. If the pump axial setting is not correctly maintained, it will see efficiencies drop below those of an enclosed impeller.
This looks like another of those postings that are started and not followed up by the original poster when further info is requested to help with their question.
Hopefully our time is not wasted as someone else might benefit from the information supplied in the follow up.
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