Help with Fans

I only have 1 right now what is more important a fan blowing In or blowing out?

You need a fan oscillating back and fourth and having the plants sway easy. Then you need a fan to blow out used air. In veg I use a 10$ fan and just hang it from top where it blows hot air out top hole in tent.

It is always better to pull air through something like your grow tent, air cooled hood, carbon filter vs. trying to push.

If you have to choose between an intake or an exhaust, I would use it as an exhaust, but allow a space for fresh air to be passively pulled in, as long as your not allowing lightleaks during flower.