Assuming whichever you choose the other is normal size for your frame.
Have you guys ever seen those who have big biceps but no tris? It looks like shit.
When you are standing relaxed the tricep carries your arm look. The bicep is in the position in which it looks the thinnest (extended) and it is only really visible from the front.
It's all about thinking about the relaxed everyday look. How often do you flex your biceps in public?