Are all OpenPGP public key server equal?

Most OpenPGP key servers are integrated in the SKS key server pool and exchange keys among each other. You can have a look at the pool status pages to get an overview of the contained servers. pool.sks-keysevers.net resolves to a (weighted) random choice of those servers.

Actually, keys.gnupg.net is a simple alias for pool.sks-keyservers.net (technically speaking, a CNAME to this domain):

$ host keys.gnupg.net
keys.gnupg.net is an alias for pool.sks-keyservers.net.
[...]

In the end, it does not matter which server you choose, as long as it is contained in the pool. Using pool.sks-keyservers.net is a safe choice, and keys.gnupg.net is equivalent.

A typical answer of a key server that is not synchronizing is the PGP Global Directory, which also performs a simple ownership verification of the mail addresses contained.

Key servers exchange keys. That is why it does not matter which server to use.