The heat from that GPU is not much, doubt it will be a problem but I'd wonder how much of the motor's magnetic field is polluting the ground plane of the card, so would do a wifi benchmark with the card in both slots, and the video card at different loads to vary the magnetic field frequency.
Then again if the video card fan exhausts directly onto the back of the wifi card on the area opposite the heatsinked processor, it either is, or isn't (lol, see below) going to help it either, so seems simpler to use the other option of more distant slot.
Then again the opposite could be true, if the exhaust air temp from the video card, is cooler than the back of the wifi card from its own heat and passive cooling, then it could actually cause it to run cooler than if more distant.
Get an IR thermometer and measure, though having it further away would tend to make the video card run cooler, or wear fan and build up dust slower at lower RPM to achieve same temp.
Then there's the other option, add a fan. Put one on the side panel, or even just mounted on the case floor (not a hole in the case floor, perpendicular to it), with no external air intake, blowing across the cards.