An iPad Mini, because it's an iPad, has great support for Microsoft Exchange, in addition to IMAP and POP servers. If your company supports iPad access to corporate resources, your iPad Mini becomes just another iPad for both your company and you, giving you the most security of any mobile OS outside the BlackBerry, as well as the greatest selection of effective mobile productivity apps. If you hadn't installed those apps on your iPad Mini, you can download them from the App Store at no charge if previously purchased for a work iPad. The only real difficulty you might face is dealing with the smaller screen and thus smaller keyboard for any text-intensive work.
The Nexus 7 is your second best bet for doing work from a media tablet. Its Android 4.1 "Jelly Bean" OS has solid security capabilities and Exchange support, its Email and Calendar apps are solid if unexceptional, and Google's Quickoffice HD Pro app for Android is capable enough for most business work. Plus, as with the iPad Mini, you'll find apps for a wide variety of business needs, from Salesforce.com to SAP access.
Of course, many enterprises refuse to support Android devices due to concerns over its malware-infested Play Store and Google's history of inattention to security. So even if your Nexus 7 or other Android tablet can help you out in an emergency, your company may or may not not let you use it.
The Kindle Fire HD supports Exchange, including the same kinds of security policies as standard Android devices -- a new capability in this second Kindle Fire generation. The Email and Calendar apps have simpler UIs than the stock Android versions, to fit better on the small screen. But all the capabilities you need are there, including attachment previews and calendar invites. I was impressed with their quality given the Kindle Fire HD's decidedly nonbusiness target user. It too can be used in a pinch -- if your business is willing to let it in.
Although the Amazon Appstore is curated, the Kindle Fire HD allows sideloading of apps like other Android devices do, so you can install non-app-store apps on it. A basic version Quickoffice is available for the Kindle Fire, so you can do basic Office document work with it.
The business connectivity winner. In all cases, assuming you're permitted Exchange access from your media tablet, you have basic email, calendar, and contacts capabilities available. But to do real work routinely, your best option is the iPad Mini.
Deathmatch: SecuritySecurity is probably not top of mind when choosing a media tablet, but it should be one of your purchase criteria.
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