Personally, I would go with the 840 Pro. The 830 is an older model that is being phased out. If you can find a fantastic clearance price, then it might be worth purchasing.
I maintain the ssd database listed in the sticky at the very top of this forum section. Here is the link:http://www.johnnylucky.org/data-storage/ssd-database.html
Scroll down to the Samsung section and follow the links to technical reviews of the 830 840, and 840 Pro.
BTW - Samsung issued a firmware update for 840 Pro that improves performance.