NAT44 translates only client addresses, leaving server addresses intact, NAT46 has to translate both client and server addresses, which can be taxing if there are more servers than clients behind NAT, which is further exacerbated by server farms as each domain now has several addresses. Well, if clients connect only to facebook and google, that's only two addresses to translate.

I don't think you can use port numbers to disambiguate between servers as clients will connect to port 443 for https.