My vote is for the Synmat. As you mention, it's lighter, wider and packs smaller.
The two advantages of the Q-Core are that it's thicker and more insulative. However, I'm not sure being thicker will translate into an overall more comfortable pad, because once you're thicker than 2.5" I find a little extra width to be nicer than a little extra thickness. 3.5" is sweet, but it seems pretty darn narrow to be that thick.
The Q-Core is more insulating, but going from R-3 to R-4.5 kinda falls into no-mans land. Obviously more insulation is a good thing, but pragmatically there's not much you can do with an R-4.5 pad that you can't do with an R-3. What I mean is that an R-3 pad is good for 3-season use, and if you want to get into true winter use then you want about R-6. So there's a pretty narrow window of time where R-3 is insufficient but R-4.5 does the trick. It's still a good thing, but not a huge deal. You want a DownMat UL 7 if you want to use it year round (R-5.9).