[krify]

[B]Cookies[/B

Pros:
Legacy support (it's been around forever)
Persistent data
Expiration dates

Cons

Each domain stores all its cookies in a single string, which can make parsing data difficult
Data is unencrypted, which becomes an issue because...... though small in size, cookies are sent with every HTTP request
Limited size (4KB)
SQL injection can be performed from a cookie

Local storage

Pros

Support by most modern browsers
Persistent data that is...... stored directly in the browser
Same-origin rules apply to local storage data
Is not sent with every HTTP request
~5MB storage per domain (that's 5120KB)

Cons

Not supported by anything before:
IE 8
Firefox 3.5
Safari 4
Chrome 4
Opera 10.5
iOS 2.0
Android 2.0