First option is more natural and strategic wise more sound but problem with that is russia can not deliver numbers and structure of their selling agreements.Remains to be seen if it will be implemented.
It is still widely believed that Iran is looking firmly towards Russia for a revitalization of its Air-Force although China would be the only other alternative.
Now how willing the Chinese would be to draw the immediate ire of Americans by providing Iran with advanced jets is not known but to be honest. They aren't going to start sacrificing billions of dollars in potential sanctions just to give Iran some jets, in my summation.
If they are willing to sell you air fleet assets with local production i would go with them if not, China is more rational choice until you aviation industry matures enough for domestic solutions.
Russia is not reliable partner and beside fact that you extended them help i do not see that they are in rush to deepen defence relations with Iran.