Already answered above, but either is fine. I have low BF so I get by 31 gauge 5/16 inch just fine for any peptides. I don’t ever do shallow IM injections with peptides, so I just steer clear of longer skin pins.
The only thing I will use a 1/2 long needle for is if I using a 27 or 28 gauge to inject some oils shallow IM into my delts or glutes. Some people may not be lean enough for it tho. It beats the shit out of using a 23 or 25 gauge 1 or 1.5 needle any day one you can do it.