I have a little dilemma to resolve. I am about to submit the major paper of my current post doc after working for 3 and a half years on this. This was a major collaborative effort involving people across two labs so the paper has 9 authors, currently myself as first author and two equally contributing senior authors. I have spearheaded the project since funding acquisition (I've got a Marie Curie grant for this research) and it will really be the major paper with which I would ask for new fundings and try to potentially start up my research group.
The paper is finally ready for submission and I was asked yesterday by one of the senior authors if the current second author should be upgraded to co-first (I would still be first name and corresponding author together with the two seniors). If one looks at mere contribution (i.e. time spent working on the various experiments and data analyses), I've done way more work than any other person (it has always been my project so others helped but I worked 90% of my time only on this), and all of the project coordination, but the second author has contributed very important analyses and is helping substantially in manuscript writing. He has always had a super nice collaborative attitude and we worked closely for several months on the main experiment (main because it gave the coolest results, not because it involved 90% of the whole project, although it was still 4 months of solid work just to collect these data), but I do feel it would have been different (more work load on him also for this main experiment and more requests to work together also on different components of the study) if we had decided this at the beginning and not now upon submission.
Anyways my question is, does it make any difference for my future career or how this will look like from the perspective of an employer or for the community of researchers working on the same topic, whether we are co-first or not? Because if it does not change anything to me then I do not see any issue, but if it somehow would give the idea that I was not fully independent on this (in terms of developing the ideas, coordinating etc), then I would be a bit more reluctant as I am trying to establish some "leadership" recognition in the field with this work. What different thoughts the two scenarios would prompt on someone looking at the publication list on my CV for example?
Thank you all for your answers!