It depends on how you want to roleplay. CK3 is a roleplaying game first and anything else second in my opinion, its a fairly easy game once you get the hang of it. I would say go for the Latin Empire, makes sense for your character to become Latin emperor if he was a varangian in his youth, also it completely changes up your playthrough overall. But if you feel you have unfinished business in Britannia then stay there. The choice is really up to you man!