The good thing about the SX2 is that it works with Serato & Rekordbox were the RX only works with Rekordbox, so if you use Rekordbox and don't like it, then you are stuck.

However the SX2 allows full use of Serato, but you would have to buy Rekordbox, where the RX comes with full access to Rekordbox. With either option you will have to purchase any expansion packs you want to use.

My recommendation is to buy the SX and to try serato DJ cause with this software you are independant from hardware platforms. Means with recordbox you are sticked on Pioneer.
Sure Pioneer DJ is the world leading hardware manufacturer but there are also others on the market - Serato offers a lot of different hardware platforms that are compatible with this software. Rekordbox is limited to Pioneer hardware (as far as I know).

Definitely buy a DDJ-SX2, Rekordbox is a poor excuse from Pioneer to create a niche market for their controllers and force you to use their crappy software. I never understood this path they are taking until I found out PioneerDJ was sold to an investment company well known for their "how to make a lot of money quick" schemes.

They thought they could capitalize on making hardware that exclusively works with their own proprietary software, kinda like NI does with Traktor and the Kontrol controllers. Difference between the two is that NI listens to it's community and don't dick around if you want to use your NI controller with say Serato.

The New Pioneer just wants to make money and I'm not buying into paying a lot of cash for a controller that can only work with their product, if they for any reason discontinue Rekordbox (not likely) your stuck with a controller that can do F all.