* You are probably not Google and the http/https server is almost certainly not a performance problem worth worrying about.
* Swiss army knife. At my day job we use a pile of Apache modules; if we went nginx we'd have to replace these with a kitchen drawer full of standalone gadgets.
* You know Apache well.
* Lots of LAMP apps pretty much presume Apache (WordPress plugins in particular) and nginx support is a me-too.
* Smaller and cleaner config.
* You know nginx well.
* nginx support is getting better, if you like nginx it's worth checking your prospective application.
Honestly, for almost all use cases it probably doesn't matter which. Pick whatever makes you most effective.
Fixed in Plus, not in the open source product.
And unless you support it solidly and long-term, nobody would really use it: paid support from Ngnix is much less risk.