Pens simplify the number you log
A pre-set pen device typically delivers a fixed dose per click or setting, which makes the “dose” field in a log entry simple: whatever the pen was set to. There’s no reconstitution maths involved, since the manufacturer has already fixed the concentration.
The trade-off
Pens offer less flexibility if you want a dose between the pen’s fixed increments, and they’re generally sold at a higher price point per mg than vial-and-syringe setups, reflecting the added manufacturing and device cost.
Vials require more maths, but more control
A vial reconstituted at home requires calculating units from mg and concentration for every dose, which means more room for a logging or calculation error, but also more flexibility to use any dose within the vial’s range, not just fixed pen increments.
What this means for a log
A vial-based log benefits from recording the water volume and concentration once, clearly, alongside each week’s specific unit draw, since the units drawn will vary if the dose changes across a titration schedule.
Which to choose
Neither is objectively better for logging purposes; a pen simplifies the entry itself, while a vial gives more flexibility at the cost of more careful record-keeping. Choosing based on cost, availability and personal comfort with the maths is a reasonable way to decide.