test drove a 2015 Q50S when i was looking at my 14 IS350. It was close. I did not like the interior of the Q50S as much as the IS. As aggrevating as the IS's nav interface was, the Q50's dual touch screens were just as bad. handling was a little bit off, and while it was faster, it wasn't by much. With the updated engines from the MMC of the Q50 (3.0tt V6) , my opinion may change now however. They also supposedly fixed the handling issues as well.

but if you aren't looking to buy new, I would lean towards the IS.

 

i was debating between IS F sport and Q50S. I just couldn't stand the interior on the Q50S. Also, I'm not too fond of touch screens in general and touch response wasn't up to my expectations. I'm a big fan of their engine (i owned G35 before) but that wasn't enough for me to sign for Q50. My biased opinion is IS over Q50.

 

I compared them and ended up with a 350 F Sport. As others have said -- build quality, touch screen. I also don't like the paddles on the column and prefer them on the wheel. I found the seat more comfortable in the F Sport and the brakes also felt more responsive to me.