It is often said that if you are in a group of explorers hacking through a thick jungle, the manager is worried about cutting a straight and efficient path, while the leader is climbing the trees to make sure that you are goin in the right direction. Fact is, you need both.
When I describe architecture, sometimes I need to lead. Sometimes it is about insuring that the direction is the right one. I need to make sure that we are keeping the correct things visible as the goal, and staying focused on the elements that will get us there, while staying tuned to the "snares" that would prevent progress.
Other times, I need to manage. I need to write the document, create the diagram, lead the team meeting, enter rows in the schedule. It's day to day, block and tackle stuff. It's taking my turn at point. It is not creative, but it is necessary.
This distinction applies whether you have direct reports, or you are in a position of influence (as I am these days). The rules really aren't different, even though the balance of motivations are.
The toughest part, really, is knowing when to lead and when to manage.
I have no real 'formula'. I will say this: normally your team will tell you. The key is to listen.
Your team will tell you if they feel blocked, or confused, or if they have flipped the bozo bit on someone. They will tell you if they need help... if you know how to listen.
Listening is an art. It means not only hearing status reports (or excuses), but digging deeper to understand priority conflicts or to detect gaps in tools, abilities, or confidence. It means knowing when to express support and when to offer alternatives. It requires patience, insistence, and, as often as not, silence. Listening means going beyond the "what" to get to the "why," and then to push to the "why not?"
I spent a few years as a manager. I'm no grand expert. I will say this: to decide whether you need to lead or to manage, the key is to listen.