You can go for "Pay per Use" to Quantify your needs then you can decide for 6 months plan or other plans that suits your need. And by quantifying the need i mean: What size of Database? Do you need a 50 GB DB or 100 GB DB. That's one of the question
that you can try and find the answer. Then you may also know want to know what should be the size of your Azure Web/Worker role. Also Number of instances of web/worker role. And questions like this..
Yes, you can have just one subscription for both. A subscription is like a "phone plan" - it comes a rate for each type of service (e.g. you pay $0.02 / minute for day minutes, and $0.01 / minute for night minutes) and with some included amounts (e.g.
you get 500 day minutes per month, and 2000 night minutes). Azure subscriptions work the same way, but for Azure resources. What resources you choose to use against a subscription is entirely up to you!