@aizi
regarding your question
1, doing the both is the best, first you upgrade FA then YA follows up.
2. if you mean effect to ur club, I will have to say,
You currently have 70k stadium, you are still very far base on the maintenance fee this buildings gonna cost you. You will get bankrupt
I don't agree with @joost, look at his club, he has FA1 and YC10, and is building up his stadium 11. And he told you to build YA first, you are not even at his level.
but if you can ignore me right now and follow the step @joost is developing his club then you are on right track, and that's how its Done.
for instance, I will be starting my stadium 11, in just few days, but am still considering downgrading my YA/FA to a point that I will maximize all the profits I can get every season to gather money for the strategy @carasco just mentioned FA4/YA5 and this gonna take me long long time, even with the stadium 11..