If I pay $5 for the 120 credits, and then run out of credits, will it then allow me to just earn sky credits through watching ads, instead of paying for more credits.
Yes; once you purchase the pay as you go plan for five dollars and use up the 120 credits; you will be able to earn credits and use those.
If I purchase the $5 go plan could I skip the line
is it should be the "pay as you go" plan or any plan There is no difference ?
Yes you will be able to skip the line
It does not matter if you purchase pay as you go or a subscription plan. Although at the moment the billing process is a little buggy, but a fix should placed as soon as possible.