My requirement is that, i need to upload data from an excel sheet to an existing sharepoint List in SP 2010.I will have 2 different Lists Like this. Then I need to compare Both the List and retrieve the rows that does not match and save it in another List.For Comparing, I will have a Common ID field in both List. I also need to keep a version for the 3rd List data, may be need to export to an excel and save it in a Document Library.
Assuming you have Excel Services you can access the data in the Excel file using the Excel REST service. There is a basic intro to this on my blog http://paylord.wordpress.com.
You can also use REST to get the data from the list.
I would use jQuery for the grunt work of getting the items, comparing them, and writing data back to another list.