From a Plex perspective, they both will have basically the same performance in terms of HW Transcoding capability.
You can get into religious wars regarding Synology vs. QNAP.
I have used both, and prefer QNAP myself for a couple reasons. They tend to have beefier and newer HW support than Synology. They also are running based on newer Linux kernel versions (4.2.8 for most x86/x64 models) than Synology.
Synology has generally had the cleaner SW interface/apps in the past, but QNAP has closed a lot of that gap.
So I tend to land on the general concept that SW can always be changed/improved (especially with NAS models now supporting Docker & VM’s), but HW you are stuck with until your next NAS upgrade.
So where possible I would tend to lean toward bigger/beefier NAS model than you may think you need, as it is a lot cheaper to grow into it vs. outgrowing a smaller one quicker and then having the cost of a 2nd NAS.
Generally the smaller NAS models like the x53Be series use SO-DIMM’s (not full size). According to QNAP specs, they use 2 x DDR3L SODIMM slots (must be matched pairs for dual dimms).
I would verify the speed of the existing DIMMs, but you are probably looking at say DDR3L SO-DIMMS in the 1600 Mhz clock range. I have generally used Kingston HyperX family dimms in all of my QNAP’s.