I went through this already. I use cdj1000mk3s with an Audio 6. Yes there is latency when you use the cue button on the cdj but there isn't any latency if you use vinyl mode and release like a record on the cdj.

The Audio 6 is exactly the same as the SL3 in terms of ins/outs. SL4 and Audio 10 are the same as well, well almost.

Here's what I'll tell you - I prefer Serato's software/gui but think that the Native Instruments hardware is actually better - it seems to be louder and just built more like a studio grade interface - i know that all sounds subjective. oh yeah, i've owned both.