I have the DDJ-SX and totaly love it and use it with Traktor and in many ways it like using a CDJ2000/900 DJM800-900 setup. I also want to get a XDJ-Aero because of rekordbox intergration, When I go to a club that has CDJ's I only bring my Macbook.
With the DDJ-SX you get something that looks and feels like a club setup but you need a computer and a software, software that is not rekordbox compatable, so if you want to play on a cdj with only a USB stick you need to set everything up again in rekordbox. There is a realy amazing program called Rekordbuddy for Mac that can transfer you liberary from Traktor to Rekordbox, (They have talked about making it Serato DJ compatable too), but this still means some extra work.
With the XDJ-Aero you get something that is a bit limeted compared to CDJ2000/900 and DJM250 but you can use more sorces and you can use the same liberary at home as you can in a club, need to bring the XDJ with you, you only need to bring an USB stick or your phone witch you would bring anyway.
The Aero is smaller and works without a laptop, The SX is big(realy big) but not realy that heavy, it waights less then my VMS4 witch is 3/4 of the size. Bringing a laptop is not that much more and only 1 extra usb cable.
Advantage of getting the Aero is Rekordbox witch makes it the same as using pioneer CDJ's
Disadvantake it's display is small and no waveform readout, limeted to whats on the device (FX, sampler, 2 channel)
Avantages of DDJ SX works with any DJ software(that is midi mappable) looks and feels like a club setup, Limeted to the Software you use.4 Channel standalone mixer
Dissadvantage needs other devices to be connected to work,(Laptop, CDJ's, Turntables, mp3 player) no FX whitout DJ Software.