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... in most of these scenarios is ... how much value does the spec you are surrendering have to YOU (at this time)?
If you already have a quintet of starters running sub-4 ERAs ... who are under contract or club control for multiple years, and you have two or three young arms on the farm ... then you NEED to find a way to get value for *EITHER* the young arms in the majors or the young arms on the farm. 
One subtlety is ... you don't deal from surplus ... you deal from "perceived" surplus. 
One other variable for the "rental" deal is how many draft picks can/will the rental add to your plate next year.  Tampa drafted what? a third of the first 50 picks?  They did it in large part by doing 1-year rentals.  The 1/2 year rental can be even smarter, in that you already know the production level for 1/2 (or 2/3) of the year, and can better judge the final draft pick tally you may or may not receive.
So ... trading away the 'juicy' prospect for the rental isn't always that simple.  It's trading away the juicy prospect for the rental AND the additional draft pick(s) ... (to in theory replace the juicy prospect).