I'm not on the 777, I'm on the 744 so my advice comes with a health warning.
Life at BA is no bed of roses, be under no doubts that you will certainly be working hard on the 744/747. I know that on Longhaul you do not do the x sector days you do at EZE but BA have got rostering down to such a fine art, despite Bidline, that you will probably find your monthly/annual hours at EZE and BA will be much the same, and will often be doing 6 trips a month, some possibly "back to back". Couple that with the time zone changes and night flights and you
will be fatigued for a lot of the time, albeit in a different way to that in Shorthaul.
The other issue you might want to consider is time at home, particularly if you have kids. Long slips downroute can be great but remember you're not getting home every/most nights, for some an plus
for others a minus
.
(edited to make more sense.....)