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Huge price difference but also wondering from others with experience, which would you recommend?
If you are interested in the science behind your question, the answer lies hidden in your question. You mentioned "is the extra 1/4 inch really needed".
As do most people, you translated this in your mind to "I know how little stiffness there is in a wimpy piece of 1/4 inch plywood, so how could it really make any difference".
The stiffness of a piece of material is only determined by simple addition in a case where the two layers are free to slide against each other. If just laid one on top of the other with no connection between them, two 1/4 inch thick pieces of plywood form a combination that is merely twice as stiff.
In the more common case of well attached or glued layers (or in this case just a thicker single sheet of plywood), doubling the thickness of a sheet MUCH more than doubles its stiffness. The 3/4 plywood is more than twice as stiff as 1/2" plywood.
If you are installing the new flooring perpendicular to the joists you will be ok with 1/2" subfloor but if you are running in the same direction the joists you will need to replace the subfloor with 3/4" t&g plywood