I have a task where I need to merge two separate instances of Magento into one multisite. Ideally I'd also like to upgrade them to 1.8 (currently 1.5). What I'm asking for is advice on how to move products (with attributes), customers, and order data.

I'm thinking the API may be a good way of going about this on a maintainable basis. Whilst the new site is developed, the existing ones can keep running and have the data moved when it's needed. I'm not sure how feasible this is for products however due to the attributes and store associations.

Would it be easier to do complete exports of the data and write some scripts to handle the import?

Any help is really appreciated.