For IAAS providers, you should care about the uptime of the regions (datacenter) that you are planning to use and the uptime of the specific services that your vms will use within those datacenters. In particular, I recommend paying close attention to the monthly/quarterly uptime trend in addition to aggregate uptime numbers.
And if you really care, try and find out the datacenter(s) your providers will be using and find out the historic performance of their different infrastructure and redundancy providers, and how often they actually test and upgrade their systems.
Look at the difference now when I filter by region and add the region data to the chart... it clearly shows just how hard these comparisons can be:
Provider Uptime Outages Regions Total Downtime
---------------------- -------- ------- -------- --------------
Google Compute Engine 100% 0 1 0 hours
Amazon EC2 99.9984% 7 3 0.13 hours
Microsoft Azure VMs 99.942% 56 6 2.89 hours