I vote thick end plate. It should result in a stiffer joint less influenced by prying forces which can be tough to predict accurately. The material cost of either the bolts or the plate should be fairly irrelevant so long as commonly available products are specified.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
You could use the thin plate procedure when you're not in a high seismic area.
I'm not sure that I've ever used it for new construction. But, I know other people have... my impression is that it has been used extensively in the metal building industry. So, if you're doing engineering work on an existing structure you might not have a choice....
I agree with joshplum,
thin plate used extensively in PEB structures, all steel companies try to decrease weight in every part in theses structures, however the weight of end plates will not make a problem in building weight but it is their philosophy to decrease the weight
other steel building use thick plates
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It's the thick plates connections that have been tested per AISC 358 and have proven ductility. That may not be intuitive, but that's what the testing tells us.
Now, it may be just that no one has done this testing on the thin plate connections. Or, it may be that it may be the bolt behavior leads to less reliable ductility. I can't say for sure.
Thin plate is more flexible and allows the end of the beam to rotate. Thick plate will resist rotation and create a resistant bending moment at the end of the beam. You are, as structural engineer, in need to select the type of the end plate for your considerations. Additionally you need to show that the selected design is validated and safe under all loads.
Sometimes engineers, this is mainly used by the elderly engineers as trick, select the beam size with free in bending rotations at ends, and put thick end plates instead of intended. However this trick does not work all the time. In case there is nothing to take the end bending on the other side, the column is forced to take the bending and rotate, sometimes this may cause fracture on the column. So take care on the details. Your question is very basic, and you need to know the answer without asking anywhere.
I propose the selection must reflect the application, though in AU a number of relatively authoritative guide(s) (ASI, though I cannot recall the particular guide (s) with certainty) recommend that base plates that are thick, in preference to thin; to minimise cost.
