Great question, and the answer is: It depends.

IM has the benefit of being less frequent as you can inject larger volumes into muscle than you can into fat. If this makes a patient more compliant with their regimen then that's a good reason to go with this. Twice weekly on cypionate using this provides a relatively stable T level. For this we would usually go with 21g draw/25g injection/1cc or smaller syringe (to better judge dose), and needle length of about 1". That said, using smaller needles are perfectly fine as they will cause less damage/hurt less/and cause you to inject slower (which is good for you), but most guys don't like it taking a long time. The 1" can be smaller as well, as long as you're getting solidly into the muscle. Skinny guys with little fat may not want as long of a needle & be just fine with smaller.

For SubQ this has the benefit of providing a more even release of T into the body. You'll need to inject more frequently as you can put less in at a time, but that combined with the fact absorption through fat tends to be slower means it's more of a time release. This is great for men who don't want AIs but who have shown a higher level of estrogen conversion. It also tends to hurt a bit less. For this we would go for 21g draw/27-31g injection/0.3cc-0.5cc syringe, and 0.5'' or 5/16th'' needle length. Some syringe combos at this size don't allow for swapping of the needle between draw & injection, but this is generally fine at that small size.

In general we start men on IM due to needing to do it less often & consuming less supplies, then adjust if it's needed or they just want it.