When I built this box in my signature, the branch of choices you laid out looked similar to mine. Though I was looking at the C2750 (8core version).
My needs were similar, but I also wanted do do some virtualization of a few preexisting servers I've got.
I loved the idea of the Avodon boards, It seems like they are built with the NAS appliance in mind. the build cost would be fantastic considering you don't have to buy a CPU or an extra SATA card. But in the end, it comes down to cpu horsepower. While others may disagree, the ATOM chips just aren't there yet for what we're asking the xpenology boxes to do. I'll again reinforce this is for me, but this is what I found in my research.
http://www.cpubenchmark.net/cpu.php?cpu ... Hz&id=2331 <-- notice the single thread performance. Not all processes are multi threaded. Loading up one cpu intensive task on a single core can be a killer here with this chip.
http://www.cpubenchmark.net/cpu.php?cpu ... Hz&id=2230 <-- now look at this single thread performance. One core has nearly the same power as the entire chip. Of course you can say you can't compare Atom to i5, but we have to, this is what you're asking.
My experience over the past few years with Plex tells me don't under power it. Plex is great, like the ultimate swiss army knife, it'll do anything well, some things great, others are a compromise. But the one thing that stands out, is even if you don't think you'll be transcoding all that much, you will transcode more than you think. The variety of devices and screens we ask plex to serve to demands it. Even if on paper it should be able to direct stream, sometimes it forces a transcode.
My choice was a bit more clear, i knew i needed and wanted the power to do multiple streams, and multiple VMs. I needed the high single thread rating and multiple threads. Of course I don't have an that C2750 to compare, but numbers don't lie. I'm sure it does fine, but throwing more cores at an atom only makes it wider, not more powerful. I'm of a mind to spend a few dollars more and overbuild, having built servers for a living and countless PCs this is my accumulated opinion.
I really want those Atom boards to work, but they just don't have enough horsepower for all the things xpenology can do. I know others will protest, but again the numbers don't lie, just make sure the numbers are right for your uses.