Ben, to be quite blunt – yes there is a big difference between just going for 14 days vs. 28! Same way why many people prefer 45 days over 28 or even 35 days. Simply stated the longer it ages the more moisture is released thus concentrating the beefy taste plus the tenderization that aging causes.
As for starting with prime vs. say choice yes that will just be that much better. OTOH since there is loss due to trimming some people will feel that merely adds to the per pound cost and they stick with choice. BTW I would not use select, but stick with choice. Some butcher shops refer to their better choice as being premium choice. Ron