Decoupling cap: Closer to chip but with via or farther without via?

Don't stress too much its all about minimizing that inductance. That doesn't always translate into distance. If I were you I would take steps to minimize all the contributions to the total path inductance between the pin and the cap. You don't mention what speeds your chip is running at but you do say it's in a QFN. I only say that because sometimes we get obsessed with adding decoupling when the package itself is a limitation.

So how crazy do you want to get? Lets minimize each section. Starting with the caps you could pick a lower inductance package for example a 306 (603 turned sideways), 201s if you can get your values, MLCC caps, or there's an X2Y variant made for decoupling and RF-land.

Next the mounting strategy, if one via is good why not two. More parallel vias should be a lower impedance. If doing 0306, or 201 style caps make sure to do the via to the side trick, again trying to minimize loop area.

Ok so now I say put them on the top. Make part of your top layer a copper flood for the power side. Then on the next layer 5 mil or less below the top make that GND. Use multiple gnd vias at the socket pins. This will give you a nice low impedance path from the above caps into those pins. I did an analysis one time on HS section of an FPGA. A nice tight plane structure and caps like I described outperformed capacitors directly underneath the parts using multiple vias.

Finally if you want to feel better about it you could do some simulation or analysis. There's plenty of topics written about PDN design out there. If you don't have a simulator check out Altera's free PDN excel tool. The design guide has some really nice information in it.

I've used those sockets before they're pretty nice, and have also stressed about where to put caps.

I would say the via solution is the better one. However since you are using a socket I expect that the socket dictates (deteriorates) the overall performance (inductance to a decoupling capacitor) that in the end it probably doesn't matter what you do. The via or the long trace.

But if the via solution is acceptable (also regarding thermal issues) then I would choose that.

If the space is available you could also just place the pads in both places and then later decide or measure which solution is better.