A bit off topic, but if you want to further quieten your downstairs PC, you might like to consider swapping it's HDD to a solid state drive (SSD) which have no moving parts. SSD's cost a bit more that mechanical HDD's gigabyte-for-gigabyte, but for silent operation it might be a price you are prepared to pay. At time of writing a quick search at Amaz0n reveals 120GB SSD's available for less than GBP 50. Not huge by comparison with current multi-terabyte HDD's, but more than enough for hosting the OS and a bit of transient data. One caution I'd sound about SSD's is that a while ago, there was much talk about ensuring OS supported "TRIM" if you want to use SSD, to even out the "wear" on them. I'm not sure what the current status quo is with this, but I'd sure a bit of Googling "SSD + TRIM" will reveal all if no-one here knows.
With regard to the original question - are you saying you have HomePlugs currently, or are you considering purchasing them to address your streaming requirements...?
I would tend to favour HP's (even though I've never used them) over wi-fi (and of course ethernet over HP's) as wi-fi tends to be a lot more fickle and performance of wi-fi is much dependent on the number of wi-fi devices in your locale (yours and the neighbours) and traffic levels which mean it can be great one moment and poor the next. Wi-fi, I'm afraid, "is just like that." The more data you can shove onto your HP/ethernet links, the more capacity (think of it in terms of "air time") you leave available for your remaining wi-fi devices to fight over.
The "size" of BD rips doesn't matter when you're assessing networking requirements - it doesn't matter if they're 1GB, 10GB or 100GB (though of course that's very much a consideration when sizing the storage requirements for your NAS.) What matters is the bit-rate requirements of the file when they are being streamed. BD is currently the highest and received wisdom suggests they are of the order of 30-50mbps. DVD is lower (4-5mbps) and things like YouTube are lower still. A single BD stream can easily be handling by 100mbps ethernet or better. There's plenty of anecdote here that people also manage to stream BD over homePlugs, though much is dependent on the link rate ("speed") you HP's sync up which in turn depends on the quality of your mains circuit. I'd buy the fastest HP's I could afford (IIRC currently 500/600mbps units.) I've had success streaming BD over 300mbps wi-fi - though I do have very favourable wi-fi condition - only one client, no interference from neighbours, etc. - YMMV.
I'd echo the point made about one of HP's microservers - mine's about as quiet as a fridge, a boiling kettle drowns it out - though it depends on the HDD's one fits. Microserver gives you complete flexibility to choose the software environment - either Turnkey NAS freebie, or a "full fat" OS like Linux/Windows with which one can fiddle as much as one like. All can provide basic network shares and it sounds like things such as Plex can run on many platforms.
If you want something more "appliance" like (ie you buy it and it more or less "just works" out-of-the-box) you could look at a commercial NAS solution, but that may limit the flexibility you have to tinker with the software environement.
There's an FAQ about NAS pinned in this forum.