What is the point of Alfred and how is it better than the default spotlight?

If you have to ask, get 8GB, you know it if you really need 16GB. You only need that if you are doing rather heavy video editing or even gaming etc. If you are buying an Air, you are probably just doing messaging, email, surfing the web, typing up documents, etc. So 8GB is enough for that. But if you have the money then you should go for the upgrade. I would recommend the SSD upgrade first then consider the unified memory upgrade.