Thanks for the feedback. Let me answer why I'm doing what I'm doing.
CT - As you said you have spent years to get to a solution... Me too... I started with the HP EX475, so I've been trying to find the magic bullet for a home storage solution for 6 years.
To your points.
iTunes server on Synology only works for audio... NOT video, so no joy on streaming to an Apple TV or iOS device without an intermediate step. iHomeServer or Plex would be that step for me.
1. Stand alone server - recycled i7-920 that just was taken out of my workstation, with 2 x 5 bay SATA iStar hot swap cages (would be starting pools with 5 repurposed WD RE4 2 TB drives, cages and drives from previous unRAID setup), 256 GB SSD for OS and apps
2. Devices - iOS mobile devices, Apple TVs, a couple of Samsung TVs, XBOX One, 2x OpenELEC HTPCs and 1 WMC - Most clients running Plex, WMC running MyMovies
3. ISOs and MKVs - For reference quality material being streamed high bit rate 1080p to capable devices (and can be transcoded on fly with enough horsepower to other devices)
4. Other Apps - Currently this will be purpose built for Plex/Media and Backblaze, no VMs, no other apps right now. I may transition my home automation platform to the box at some point, but that will be when I upgrade CPU to something with better power efficiency, but similar trancode capabilities (newer i5 with lower TDP but high Passmark scores)
5. Data requirements - Current Synology is 5 x 3 TB WD Reds in SHR, so 12 TB addressable space... Currently 2 TB free, so some room to grow and time to wait before 6 TB drives drop a bit more. And yes either solution would have the ability to handle my needs with upgrading drives to larger models or adding expansion bays
As for concentrating on a single solution, part of my desire is not to be reliant on a single point of failure, so redundancy is a overall goal. The real redundancy in the media department will be the smaller M4V (MP4) files will be on both solutions, allowing me to back them up locally and remotely. Organization will be done using simple rsync targets and commands built into my Synology and into DeltaCopy, or using GoodSync.
On the number of streams, it is all dependent. There are 5 of us in the house, so there could be 3-4 streams happening at any time.... Or none.... With my lower quality M4V files my Plex Synology service can handle the 1080p streaming natively to most of my devices (DirectPlay in Plex parlance), but for "reference" files I use uncompressed MKVs so I can enjoy a BluRay quality experience without the disk my Synology can't handle these at all, which is part of what kicked off this science project. Also if I want to bring a movie up REALLY quick I can just rip 1:1 to MKV (20ish minutes) and go until I transcode (1-2 hours).
In short I keep MKV for high quality reference material or favorite movies, and I trancode everything to M4V at some point to keep in a smaller package, but "lower" quality.
Running both my Synology and a new home server (will have to be Windows 7 or 8 for Backblaze) will allow me to put the big server to sleep when high quality streaming and media are not needed 16-18 hours a day, and the Synology will allow steaming of the M4Vs to mobile devices or kids devices where quality is not as demanded.
Additionally, as I mentioned before I am a redundancy fanatic, so the NAS and server option gives me the local redundancy I want plus I will get the offsite backup with Backblaze. My experience with Crashplan was horribly slow and my backups (~4 TB) NEVER finished after running it for over 16 months and seeding it with a 2 TB drive and a 100/30 Mbs Comcast Business connection. The server crashed and I lost the other 2 TB, needless to say there was much cursing and re-ripping time to replace the data (movies).
Now I might end up transitioning back to a more server centric setup if I find the stability, reliability and feature set on par with what I have with my Synology. But since I already have my Synology up and going, and it does a REALLY good job with its feature set, I don't have a real reason to take it out of the loop right now. And to be honest, my need for a Plex server for on the fly transcoding and a Backblaze client was the real genesis of this current quest so I started simply by looking at Storage Spaces as a simple bridge, but since I've done my research, DrivePool seems like it may allow me to swing back towards a server approach; and since I have enough surplus hardware a new mad science project just kind of blossomed.
So to wrap up.... I think I know where I'm headed and why I am headed there, I've taken many a path to get here and most likely there will be more paths in the future.