I don't have any experience with any manufactured NAS devices (I re-purpose old PCs into NAS devices), but just looking at the two, I like the Synology because the fan in back appears to be a 92mm fan while the fan on the QNAP is a 70mm. Larger fans tend to move a lot more air at much slower RPMs, thus they tend to be much quieter.
That said, the Synology does not have USB 3.0 or eSATA support like the QNAP does - but that does not matter if you will be connecting by Ethernet (which I recommend over USB any day).
Edit add: Synology clearly states their device has a 2 year warranty. The QNAP does not clearly state anything about the warranty. It might be 2 years, but it might be 1 (unless I just need more coffee this morning).