On DDJ-RB the fact that it comes with the full record box software ( this is rare) already with the controller puts it at a significant advantage from the get-go. You're essentially paying only $120 for the controller. Whereas the beatmix 2 is $300 not including the price of Serato Dj. But beatmix runs automatically if you're on Mac, no drivers needed. The DDJ-RB you have to install, auto driver, record box... bit of a hassle.
I like the decks which are a nice metal look and feel on the BeatMix.
On DDJ-RB you have 64 programmable pads vs 24 pads on BeatMix.
ddj-rb has manual loop and doesn't have auto loop function and slicer loop .. BeatMix doesn't have manual loop but has auto loop.
Personally I would buy both but as price depends on what you need... I would go with DDJ-RB and after a while I would buy the beatmix and see which ones are closer to me and my style.