[fataliss]

You can also have a combination of both. But basically I second this post, you have to figure out what is going to increase as your product gain momentum. If you will be handling millions of requests because each client is going to do plenty of them, are they going to store data... Like if you were Netflix, you would have to consider the number of users growing, but also the size of your catalogue. Try to define what direction your business is going as it grows and then you will be able to narrow down the costs more accurately.