In general migration is much easier in my opinion. IF you have ever done migration in the past you will know the main focus points always been in a nut shell
1. Size the new exchange 2010 server ( hardware configuration, mailbox , Hub, CAS, Edge etc )
2. calculate the IOPS
3. Design SG / DB for Exchange server or Servers
4. Design or Architect the roles ( Number of servers )
4. Decide high availability ( cluster etc)
5. Implement / Execute the plan
6. Once you have your new Exchange ( new one 210 in this example) server use mailbox move wizard to move mailboxes over to it.
7. Use MS articles to make sure migration is smooth, move all required Exchange data, PF folders, mailboxes etc
8. Make use mail flow works , other services are running properly OWA, Outlook anywhere etc…
9. Shut down the old exchange server to make sure nothing is getting crazy
10. After make sure everyone is happy Un-install your legacy Exchange server and complete the migration.
As you can see from basic steps the required actions takes planning and doing your homework correctly (-:
Since Exchange 2010 is not upgrade to Exchange 2007, and it is entirely different powerful application you may want to take a bit more time to investigate some of outstanding futures, the redundancy option is one of the most powerful future ( DAG), High Availability and Site Resilience.
Planning and Deployment
High Availability and Site Resilience
Exchange 2010 System Requirement
Exchange 2010 Prerequisites
Upgrade to Exchange 2010
Deploying Exchange 2010
Roadmap for Exchange Features
Database Availability Group Design Examples