Merge contact and account, or what?

I have a fact table where each transaction is a donation. The donation can be made by a contact or an account.
My initial plan was to merge contact and account (becoming donor), but contacts can also have relationships with accounts (as a contact of some type). The account's contact can also make donations in their own right.

Current thought is to have 2 columns in the fact table - one for contact, the other for account. The non-relevant one being set to 0 to enforce correct granularity.

Downside of that is that it makes querying more complex as the user will have to check whether each donation is by a contact or an account.

My other thought is to use a view/other table as donor, but I'm concerned about performance.

Anyone have a better approach?
Thanks
Andrew