Read the comparison chart- http://usa.kaspersky.com/products-services/home-computer-security/pure?domain=kaspersky.com
Basically both are the same, Pure just has some added features which most users would never need like Password Manager and Password Sync (already free from LastPass), Scheduled, Automatic Backup, Online backup, Easy-to-Use Encryption, File Shredder and PC Clean-Up Tools (several free products already have this), Control Center (TeamViewer and several other free products have this).
These features would not be worth the extra $10.00 yearly for most users when you can get them all in free products.
The security features are exactly the same for both products and I bet Kaspersky Internet Security would be much lighter on your system because extra features means extra resources.
Enjoy!!
