I would use a slow shutter speed to 'burn in' the background and then have the flash on rear curtain sync to freeze the motion. With this technique you could use your 24-105. Expect some light trails. You'll want to use full manual mode and play around with the shutter speed and iso to have a proper exposure between the background and foreground.





you see this a lot in nightclub photography here are two examples:


http://www.flickr.com/photos/salmarstonphotography/3739785955/


http://www.flickr.com/photos/salmarstonphotography/3740708328/


The effect can be very subtle or extreme depending on what your going for.





I might also try doing a 10 second exposure on a tripod with no flash just to see the effect.


Or if the light is any good just use your 50 and try no flash with that for a while.