I'm just a simple tenno trying to make my way in the universe so... I'll just wait to see the rewards
My first instinct, though, was to join the Grineer because they're the hardest faction to fight (respect). Their pulls/knockdowns spell death and their napalms can kill you from afar through your Frost bubble.
Corpus on the other hand... they're easy to kill. Having to look up to cap an osprey is the most fighting them amounts to.