I think it really depends on what system you started using first. So, for me, it's trophies.
However, I'm a cheap arse, and so the only time I get to play my 360 on gold is when I create a new silver membership (free gold for a month)... making it harder for me to get "attached" to my achievements.
I have one permenant account, but I can only play that offline, so I'm not entitled to all the achievements.
Unless anyone wants to give me any of their codes..... yeah, I guessed not.