What it boils down to is who you trust more. Both have had their fair share of cock-ups, but of course, Microsoft has had more time to cock-up.
Gartner says Gmail's enterprise market share is just at one percent, but it owns roughly half of the enterprise email in the cloud. Considering the industry's excitement about the thrilling migration to cloud computing, making inroads like that is important.
There are certain sectors where email is very sensitive that Google will not win over in the near future. That includes places like banks which really could do with stronger security and surveillance, not less of it, as Kweku Adoboli has proved. Gartner reckons Google isn't willing to introduce that any time soon.
More importantly, larger organisations, says Gartner, complain that Google isn't transparent about what it does with your data. And that is a big problem.