I started off using the standard insulin syringes until I moved on to pens and I have no reason to go back. The pens shroud the needle and are 100% painless. Many people have a psychosomatic response to seeing the insulin needle go inside their flesh and they feel it more.
If you use multiple peptides and travel, pens are much easier to travel with as one pen can be used for multiple peptide use, basically switching out vials as needed. The medical waste, a small pen needle, takes up much less space for travel purposes as well.
Injecting into the back of the arm or in harder to reach locations is a lot easier.
Assuming you have done your due diligence and purchased a quality pen, the dosing is precise, especially at uneven/odd dose (i.e. 16units, 28units, 32units).
If you filter your peptides, there are no extra steps needed (save for adding a vent needle) when filtering into a 3ml pen vial, as opposed to filling a standard/round 3ml vial.
Finally, some folks have terrible vision and reading the lines on an insulin needle might be difficult if next to impossible, while pens have large Arabic numerals.