While looking for the best approach for upgrading i came up with this content and wanted to share it with others, in the reference links more details are present, with Pros and Cons of each approach and techniques.
Choose an upgrade approach
There are two basic approaches to upgrade: in-place and database attach. In addition, there are various techniques you can use to combine aspects of these basic approaches to mitigate downtime or potentially improve performance.The following table compares the in-place and database attach approaches.
|In-place upgrade||You can install SharePoint Server 2010 on the same hardware. You can also upgrade the content and settings in the server farm as part of a single process.|
|Database attach upgrade||You can upgrade the content for the environment on a separate farm. The result is that you do not upgrade any of the services or farm settings. You can upgrade the databases in any order and upgrade several databases at the same time. While each database is being upgraded, the content in that database is not available to users.|
For more information about how in-place and database attach upgrades work, see Upgrade process overview (SharePoint Server 2010).The following table lists the downtime mitigation techniques that you can use during upgrade to reduce the amount of time that users cannot access their content or to potentially increase upgrade performance.
|Parallel upgrade||You can attach and upgrade multiple databases at a time to speed up the upgrade process overall. The maximum number of parallel upgrades depends on your hardware. This technique works for either in-place or database attach upgrades.|
|Hybrid approach 1: Database attach with read-only databases||Lets you continue to provide read-only access to content during the upgrade process. For this approach, you set the databases to read-only while the upgrade is in progress on another farm. This method reduces perceived downtime for your users.|
|Hybrid approach 2: In-place upgrade with detached databases||Lets you take advantage of an in-place upgrade’s ability to upgrade content and settings, while adding the speed of a database attach upgrade. For this approach, you use an in-place upgrade to upgrade the farm and settings, and to detach and upgrade multiple databases in parallel (on the same farm or a separate farm).|
Be aware that you can also combine these techniques. For example, you can set your original farm to read-only mode, create a copy of the farm and upgrade it without the content databases, use parallel upgrade to rapidly upgrade all the user content, and then finally switch users to the new farm after upgrade is completed. For more information about these downtime mitigation techniques work, see Upgrade process overview (SharePoint Server 2010).
Another option to consider if you are facing an overly long outage window is to use Alternate Access Mapping URL Redirection with a database attach approach, so that you temporarily redirect users to an existing farm while you upgrade the content on a new farm. This is an advanced method and should not be used unless other downtime mitigation techniques are not sufficient. For more information, see Using AAM URL redirection as part of the upgrade process (SharePoint Server 2010) (white paper).
Also Find this Useful Picture for see the bird-eye view “Upgrade and Migration for SharePoint Server 2010”