I'd go for a middle pair on the 777 from JFK so you can experience F checkin, security CCR etc. We did this route earlier in the year with the 777 outbound and the 747 back - middle pairs both ways. The 777 was far more spacious. The only way I'd choose the 747 over the 777 would be if we could get 1A/K but you both need to be Gold for that.