The Heavy refers to the extra pilot in the cruise.
The big difference between the 747 and 777 is that the 777 out of a fleet of 47 only has 15 aircraft with bunks, only some of which have first class. This will increase with the 4 new orders but as all 57 747s have bunks they will be the mainstay of the Long range trips (10hrs ish +) until the 380s and 787s arrive in 2012. The 777 has some nice work in the Caribbean and SIN SYD if you like that but you wont see much of either at the bottom of the list.
The other big plus on the 747 is seeded blind lines. This means that although at the first stage your roster is blank to be filled up by rostering after everyone elses bids, there is one (long range) trip already there. This guarantees your days off either side so are perfect for that vital weekend when you're too junior to get it off any other way. The 777 just has blank blind lines so you've really got no say in your eventual roster at all.
The drag and dropping of open trips is coming in in the next few months so we dont really know the details yet.