Is publishing in a special issue of a Q3 Journal as relevant as publishing in a Q1 Journal? I want to publish a paper in mathematics in a special issue of my topic but in a Q3 Journal (i.e., ranked in 3rd quartile). The catch is that in this issue, the editors are some of the top leaders in the area, so I guess that the quality of the paper should be high in order to be accepted. Should I go for this? Or is it preferable to publish in a Q1 Journal (i.e., ranked in 1st quartile)? How would you, as a hiring committee when applying for a postdoctoral position, see this? I honestly think that the paper has the quality in order to be published in a first quartile Journal. 

For those not familiar with the terminology, Q1, Q2, Q3, Q4. They refer to quartile in which the journal is ranked within discipline. For example, see this listing of mathematics journals on scimago with the prominent display of ranking quartile.
How would you, as a hiring committee when applying for a postdoctoral position, see this? Since I've been on "hiring committees for postdocs" quite a few times, I'll just share personal experience.

1) As to me, I wouldn't care at all where you published but would look directly at what you published. As one of my colleagues in Wisconsin put it "I just want to know what his (applicant's) signature theorem is". So as soon as your paper is on arXiv, I'll have an opinion about you. Not all people are like that, of course, but given that the postdoctoral positions are often given to people somebody in the department seriously considers working with, you may be pretty sure that if your result is good, then the interested in you person will spare no effort explaining to others why it is so and that "this guy solved a 40 year old problem on which Terence Tao had only a partial result" is stronger than "this guy published in Acta".

2) It may be beneficial to have a publication in a higher ranking journal on your record later in your career (some university promotion guidelines explicitly count Acta papers as 5 points and "University of Urjupinsk Seminar Proceedings" as 0 points). 

3) The formal "publication score" is not everything. You also make professional acquaintances and get friends and enemies over time. Sometimes I choose the journal based on personal relationships (existing or the ones I would like to establish) rather than according to any ranking system and I don't think it is such a terrible idea. 

Just my 2 cents :-)