The choice of whether you or your friend should be immortal and which one should be invincible depends on the narrative and the direction you want the story to take.

If you want the story to revolve around the concept of living forever and exploring the consequences of immortality, then you could be the immortal character. On the other hand, if you want to focus on the idea of being indestructible and unbeatable, then your friend could be the invincible one.

Ultimately, the decision should be based on the story's theme and the roles you and your friend want to play in the narrative