I prefer 1.5" 23 gauge for glutes. When I use 25 gauge I really have to press down hard on the plunger... I'd much rather use a larger gauge which allows the oil to flow quicker and smoother.
Glad to see somebody else on the boards say this. Other than slin pin size, 25g seems to be the common choice, but I can feel the difference pushing the plunger with a 23g or a 25g. I'm not after a faster injection, but I am after less needle shake when I use certain sites. Trying to minimize the inevitable scar tissue as I go on. I acknowledge tissue damage can vary with needle size OR with needle shake, but some of my shots can get pretty shaky with my old man shoulders ... gotta play to my advantages/disadvantages.
@Traork1 ... you asked about needle length too. I have used 1.5" but don't see the need unless you have some fat over the muscle you want the IM shot in. I don't do big shots, so I buy 3cc syringes with any length 21g pin attached to draw, then switch to a 23g 1" pin to inject.