Google Apps has its own apps store (not to be confused with the Android Apps Store) in the Google Apps Marketplace. These applications, many also available for mobile devices, integrate with the Google platform. For example, they might allow single sign-on, or add functionality to your calendars, emails, contacts, or documents.
By contrast, Microsoft has been slow to the Web, although it still dominates the offline world. Microsoft's Office Store sells apps that integrate with its desktop Office software, not the Web Apps.
However, the Office file formats are the default standards for business documents shared with other businesses. An entire industry of developers and service providers helps companies expand on their Microsoft applications, and a great deal of legacy code is designed to work on the Microsoft platform. Microsoft's offline ecosystem isn't likely to disappear any time soon. If your company has time and money invested in this ecosystem, it makes sense to consider Office 365 for online collaboration, and to be patient while Microsoft improves the feature set.
Winner: Google is ahead with its Web ecosystem.
Which suite should you choose?
For startups and small companies without a great a deal of existing investment in Microsoft Office documents and applications, Google offers a robust and quickly evolving online ecosystem, with broad mobile support.
Google Apps is part of a wide Google ecosystem and was born on the Web. Every document--text, spreadsheet, graphic, or presentation--can be instantly published to the Web for public access, or access by a limited group of users. Data can be pulled in from Google's various other applications, including Google Finance, or from easy-to-create online forms.
If you're on the fence, ask yourself how important it is for you to edit complex Excel spreadsheets online; if you really need online access to Excel charts, complicated formulas, and fancy graphics, go with Office 365 for Small Business. Otherwise, Google Apps for Business offers the more compelling option.