it depends where you are injecting, 1.5" are more helpful to get deep for glute injections, 1" is sufficient for quad, delt,bi,tri and pec injects, 5/8" needles are also ok for delt,bi,tri and pec injections....
I use 1" for everything although I have to be carefull on glutes or I dont get deep enough. If all your doing is glutes I would try and use 1.5" but if you good at injecting and BF isnt too high you could get by with 1" if ya have em.