I use Live mail on 2 of my PCs but may main one has Outlook as it has additional features. I also have Microsoft Office Outlook Connector installed so my online diary is synchronised wherever I am. MS could improve this by syncing the main Outlook Calander with the Live one instead of just being able to access it via outlook but that will probably come soon.
If you use a hotmail account, messages and contacts will sync but if not you'll have to export to a fle then import what you need.
To summarise I see no reason to downgrade from Outlook to Live Mail