It’s obvious that a full-length zip will provide more ventilation and make it easier to access the braces of bib shorts than a 3/4 or half-length zip, but there are other things you need to consider when looking at the zipper.
Other considerations include – is there a zipper garage? That magic little piece of material that helps to prevent chafing of the chin. What about a tactile zip pull that makes for easy shifting of the zipper up and down as you ride along?
For the more discerning cyclists there’s also the option of the “invisible” zipper, that are sewn in such a manner that when they’re done up look like they’re not there and don’t disrupt the jersey’s print!