Depends on what kind of data you are talking about - generally, there is a tendency to overestimate transactional data requirements. A lot of data can really be fit into a 1 GB SQL Azure (we are a SAAS provider and transactional data of almost 20 clients can fit into that much space). Also, for some strange reason, I have seen that SQL Azure space consumption seems to be somewhat lesser than the size of the database I see on-premises (might have to do with how they handle logs, not sure). And now 50 GB is the limit, which is quite frankly, HUGE.
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However for this, you also need to consider what increases space usage - storing images, videos or other large objects in the database can create a considerable increase in space consumption. It is better to keep this kind of object in Windows Azure.
So short answer - keep transactional data in SQL Azure and non-relational data in Windows Azure. Working with SQL Azure will also keep your developers more productive since it is quite familiar in terms of programming. Treat Windows Azure similar to how you would treat windows local file storage with some added benefits (basic table structures supported).