Well, I'm buying a new MacBook Air. I don't know if I shall create a new separate Apple ID for it or use my iPhone's Apple ID.
will be there disadvantages of using the same one?
iPhone 6, iOS 8.3
Normally the best way to use an Apple ID is one per person, not one per device. Among the advantages is that it makes it very easy to sync data (contacts, photos, etc) automatically between the two devices if you want to now or in the future. If there are any disadvantages of using the same one, I can't think of them.
I agree with FoxFifth.
The only disadvantage I can think of is that it makes it even more important for you to safeguard your Apple ID and password. Using the Apple ID on more than one device means more than one device will be compromised should your Apple ID and password fall into the wrong hands.
Use the same one. I don't like making things more complicated than they need to be, so I use the same one. Syncing is so much easier and there aren't too many unnecessary things that you would need to keep track of.
Shall I use a new Apple ID for my new MacBook Air or use my one for iPhone
