If you go back and read my trt experiment it might shed some light. I always used 21g x1.5, as this is what Upjohn recommended. The theory was that oil had to be deposited as deep as possible into the muscle and asone continuos "depot". In other words get your shit deep into the muscle as fast as possible. Even the gear was named this way.Such as "Testoviron Depot". As of late I have some buds shooting with insulin pins into subQ, some just into fat and some into deep muscle. The bloods are coming out about the same. SO..as of recently I decided that all of these years of gigantic pins was over and bought some 25g. Seems like it took forever for the oil to get out of these pins so I went back to my 21 or 22 x1.5. I just don't have the patience to use 25g. I dont have any scars or ever have any pip. I think drb ruled that out years ago when he shot 10mls from an 18g and then within a minute did the same thing in the same spot. He shot 20mls into the same spot. So back to your question...if you are a newbie then go with a 25g x5/8. Those 21g look intimidating but they really do not hurt,cause pip,cause scars etc. Now I am talking about a once per week deal.
When you go to the every other day types of gears you really don't want to keep jamming those big ass pins