The first consideration will still be cost - after all, the main attraction to CFOs with cloud is the promise to reduce storage, software and maintenance costs. Google Apps can offer significant savings over Microsoft Exchange, but so too can Office365.
The key difference for me is that whilst Google Apps can work with MS Office and Windows, there is no dependency on them. Office 365 on the other hand tends to be used in conjunction with Microsoft desktop / server based software and without them you are going to run into trouble. This is an increasing issue for those looking to work from home and non Microsoft mobile devices.
Most companies have a Microsoft legacy, so can perceive the switch to Google Apps a stumbling block, because their methods of managing email, for example, have been habitualised by the methods dictated by Microsoft. The appeal of a keeping desktop applications like Outlook (and complimentary web versions) is apparent, but for me the lock in and lack of flexibility of taking that approach is a big limitation.
Another reasons I’ve chosen to focus on Google Apps is because I believe in their philosophy of open systems and ease of entry. Pretty much everything you can do with Google Apps for Business you can also do as a gmail consumer, for free, which reduces the barriers considerably. If I share a Google Doc with you, all you need as a recipient to access (or even edit) that document is a browser. No need for MS Office or any other licence. You don’t even need a gmail account. I can also export my docs in a range of formats (including Microsoft formats) and send by traditional email too if I wish.
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