Importing into datatable would be pretty faster than an external excel object.
Also as Ganesh said, if you ADO for excel- it would be much faster.
Also you can find a significant diff if you are working huge no. of records unless otherwise either method would be good to use.
Using the Excel ADO interface is MUCH faster than using the datatable. I've got a script that iterates through a data sheet multiple times. Pulling the data directly from excel into an array cut my run time by 66%. I think a lot of that is from moving the data into memory, rather than keeping it in Excel, but that's something you might consider if you walk through the data a lot.