While doing some research, I came across a Microsoft white paper that includes an appendix that details the features available in SharePoint 2010 Server and SharePoint Online.  You can view the tables to see the differences and determine which version is right for your organization.

The white paper, titled Microsoft SharePoint Online for Enterprises, can be found here: http://www.microsoft.com/download/en/details.aspx?id=13602

For quick and easy reference, here’s a shortened version of the table that lists only the feature differences between SharePoint 2010 Server and SharePoint Online.

 

COMPOSITES
Feature
SharePoint 2010 Server
SharePoint Online
SharePoint Timer Jobs
Yes
No
Business Data Integration with the Office Client
Yes
No
Business Connectivity Services Profile Page
Yes
No
CONTENT
 
Feature
SharePoint 2010 Server
SharePoint Online
Audit Opening and Downloading
Yes
No
Records Center
Yes
No
Word Automation Services
Yes
No
INSIGHTS
 
Feature
SharePoint 2010 Server
SharePoint Online
Business Intelligence Center
Yes
No
Chart Web Parts
Yes
No
Data Connection Library
Yes
No
PerformancePoint™ Services
Yes
No
Dashboards
Yes
No
Decomposition Tree
Yes
No
Excel Services and PowerPivot for SharePoint
Yes
No
SEARCH
 
Feature
SharePoint 2010 Server
SharePoint Online
Advanced Content Processing
Yes
No
Tunable Relevance with Multiple Rank Profiles
Yes
No
Business Intelligence Indexing Connector
Yes
No
SharePoint 2010 Search Connector Framework
Yes
No
Contextual Search
Yes
No
Deep Refinement
Yes
No
Federated Search
Yes
No
Query Suggestions, "Did You Mean?” and Related Queries
Yes
No
Relevancy Tuning
Yes
No
Rich Web Indexing
Yes
No
Enterprise Scale Search
Yes
No
Windows 7 Search
Yes
No
Similar Results
Yes
No
Single Site Collection Search
No
Yes
Thumbnails and Previews
Yes
No
Visual Best Bets
Yes
No
Advanced Sorting
Yes
No
SITES
 
Feature
SharePoint 2010 Server
SharePoint Online
Secure Store Service
Yes
Yes – Tenant Level
Web Analytics
Yes
No
Public Website
No
Yes
External sharing
No
Yes
Office Web Apps integration
Yes – separate installation
Yes – comes preconfigured