I have Traktor on my laptop, and Ableton Live on my desktop(tower)

I use vinyl timecodes with a pair of Technics 1200 mk2 decks. to control traktor pro 2, using the soundcard (USB) in my Allen and Heath Xone:4D

I have a Focusrite Saffire 6 connected to my tower to connect ableton, just using line in on an unused channel of my 4D.

If i use Ableton on the send/return Fx loop, there is a delay. (best I can get is matching it but a beat out)

I was thinking it might be better to get a separate soundcard for Traktor, and use line in to the 4D, and the 4D USB then being used with Ableton on my tower, as everything is then run through Ableton if wanted. ie record / fx sends etc.

The Saffire 6 is therefore lacking in inputs/outputs.

and also another reason to change ot, is that it has to be amplified significantly by my 4D to get the level needed.

Which soundcard should I get.

I would get the NI Audio10, but it is only sold with everything else that I already have.

so which soundcard is the one to get?

It needs to run on my aging windows laptop (windows 8.1 installed) 2gig core2duo, 4gb ram.
USB 2 (not USB 3 or firewire)