We find that teams based in the US tend to use QuickBooks Online, and businesses outside of the US tend to lean more toward Xero.

This is important because it means that the features QBO is building is more focused on things like US Sales Tax laws, while Xero may have more accounting features helpful outside of the US especially.

Also, accountants inside of the US are often more proficient with QBO as compared to Xero (although they should be familiar with both).

P.S. We highly recommend QBO over QuickBooks Desktop because of their robust API—you're not going to be integrating QuickBooks Desktop easily with anything.