I agree that ex-LAX flights will likely have heavier loads. That said, if this is your first CX F experience, go for the 747. Yes, the pax to FA ratio isn't as good as the 773; however, there's nothing quite like the private room feeling in the very front of a 747. (For best effect, try to get one of the seats in row 2.) Very different from a 773, which still feels like a regular plane, even though you're in F. Also, the 747 has those enormous lavatories in F, while the 773 has regular lavs.

You might also check 831 or 841 ex-JFK. There's pretty good availability right now, IIRC. IMO, JFK is a much better and more convenient connection. That's what I'm doing in August, originating in BOS. I'll be on 841, the morning flight.