One significant factor is how well either integrates with Shopify. Ideally we would like out of the box integration without having to use a paid app. (Ouch - some of the integration apps cost more that the Shopify subscription itself!)
Do either Xero or Saasu integrate with Xero out of the box? Not only for transations / sales but inventory as well? Or are integration apps like "Carry The One", "ZapStitch" or "OneSaaS" necessary?
In any case, which cloud accounting system do you use?
Looking forward to hearing your opinion.