Theres no way friendcodes are better.
First of all its much easier to remember a name, its more personable and less robotic imo.
Secondly, in game works much better. For example, now with Geo Wars 2, they keep track of the next best score on the top right hand corner and also track your Friends gamertag leaderboard. I would rather see that bdbdbd got 1m more than me and I need to beat him, than see 123456781234 got 1m more than me and I need to beat him.
And if im not mistaken, you need a separate code for each game. So say I play Galaga Legions. Using gamertags, bdbdbd is exactly the same person as in every game, but a code will be different. so it wont be 123456781234 but maybe 876543218765 say.