Both companies promise greater than 99.9% uptime, but Google includes planned maintenance outages in that service-level promise, while Microsoft doesn't. No vendor is immune to outages, but Google has a public App Status Dashboard, seen here, that details all outages lasting more than 10 minutes. Microsoft reports service-level data directly to customers, but it doesn't post that data publicly. Microsoft says Google only reports when "there is more than a 5% user error rate" whereas any Office 365 outage affecting a customer will be reported to that customer. Both companies offer credits against future fees for service outages (only unscheduled outages in Microsoft's case). We don't expect to see vast differences in terms of hours of availability per year, but Google's been in cloud provisioning years longer than its rival has and is not excluding scheduled maintenance from its SLA.