sftp -> put : How to uplaod only newer files

I have tried winscp, filezilla to copy data from windows xp SP2 machine to the sun sparc unix machine. However speed is around 120 to 130 kb/sec. I do have lot of files & with large size.
So I switched to windows command prompt & transfered few files using sftp command. Its very fast as compared to winscp & filezilla. Xfer rate is 280 to 330 kb/sec.

My problem here is : I want to upload only newer files using this windows command prompt sftp command. I did not find any option while doing put or any other alternative to do this. [ while copying using winscp I can check the checkbox for "newer files only" . ]

How can I achieve the sftp upload of new files only, from windows command prompt?