The older passports may be better than the newer ones. For example, one of my passports issued a handful of years ago is likely to last me longer than if I were to get a new primary one in say mid/late 2018. And if I get a new secondary one today, it wouldn't last me very long either, since even those no longer allow for supplemental visa pages to be put into them -- something that is an issue when it's possible for me to have multiple days in a week where I get 3 stamps per day.

Having the most recent passport can be a mixed blessing -- it can be perceived as more likely to be more valid than an older passport; or it can be perceived as less likely to be valid. It really depends on how suspicious the person examining the passport is.