Both make dependable mech. movements in the mid to entry level segment. Just by the fact that you can ask the question tells you that whatever advantage either one has its not really all that significant. ETA makes far more, and way more elaborate or mech. movements with complications like the 7751. Miyota doesn't seem to care or want to compete in this segment.
Just me but I kind of find it hard to imagine that Miyota or ETA are investing a whole lot of R&D money into their mechanical movements. Does anyone think a new movement in an Omega or Longines would make a big bump in sales? Is it even worth the risk in lost sales if that new movement has a problem? If Miyota came out with a new and better mech. movement would it matter to folks that want "Swiss Made" on their watch? If you talk quartz I would give Miyota a clear advantage with their RX, RC, and eco-drive tech over ETA.
Both Miyota and ETA have said that they are putting their R&D money towards quartz and smart watches. Which means IMO, what we have now is what you're going to see for awhile, and if you are looking at some micro garage brand using a Miyota, Sellita, ETA, or Seiko three hand movement you're not going to see a whole lot difference.
There are some fair points in there, but keep in mind that the 9015 was a brand new movement in 2009, so it's less than a decade old. There was certainly some R&D spend on that within recent memory.
The 2800 series ETAs have a lot more "seniority" but ETA did pump some money into the platform to develop the Powermatic 80 a few years ago. It's an even newer movement than the 9015. I would not personally put them on an even comparison because I value beat rate very highly and the Powermatic 80 trades beat rate for longer power reserve. It still has a few neat tricks. The Miyota 9015 was important as a new, low-cost 28,800 bph movement to compete with Swiss movements of the same rate. Most Japanese competitors, including Miyota's prior models were 21,600 bph.
ETA has also come out with the so far Swatch-exclusive sistem 51 automatic and the low-cost, but non-serviceable C01.211 automatic chronograph.
STP's all-new STP 1-11 has been mentioned.
Clearly people are putting some money into R&D.
Basically good sturdy movements. I have a couple watches in my collection with Miyota automatic movements. One of my watches has an exhibition back allowing me to see just how ugly the movement is. A dull gray lump. But for their entry level movements they give you more than Seiko. I get manual wind but no hacking. Seiko 5 give you neither hacking nor manual wind. The miyota winding mechanism has a nice ratcheting sound to it when you wind the crown. Feels solid. But the date turnover is a nasty clunky motion. You definitely feel the calendar teeth bite through the crown. Accuracy wise +/- 1 min per day. Not great but nothing that worries me too much.
Not picking on you but I see a few points to correct here.
The Miyota 8200 series and Seiko 7S26/36 along with the non-hacking/non-handwinding Orient movements are all on a slow trajectory to be phased out.
Most new Seiko 5 models (and other automatics priced similarly in the Seiko range) are moving to their newer 4R36 (= NH36-- same movement) both hacks and hand winds. In some cases, slightly higher end movements are going to the 6R15, which adds higher accuracy ratings and longer power reserve (53h) like the 3rd generation Prospex "Monster" divers, but not within Seiko 5 specifically. Still, you do see odd throw-backs. The Recraft series specifically use the 7S26 non-handwinding, non-hacking movements to feel 'retro.' I was not a fan and sold my recraft. Otherwise, you'll mostly find the older movements in models that have been in production for years if not decades. Most of these older SKX Seiko 5 models are being marketed more heavily to developing markets than to the West or the JDM market as they can be sold very cheaply at this point.
Orient is in process of replacing the old non-hacking, non-handwinding movements that can only do date (not day) quickset by the crown with newer models capable of all of those functions.
Miyota's base movements are nothing much to look at, but they're fancy in comparison to Orient! Their base movements are all business. I respect that about the Japanese. Still-- go look at the movement on a Citizen Signature with their decorated movements. It's probably not the greatest work of art in the history of movements, but it's worlds nicer than the workhorse stuff. It would be nice if Miyota wanted to go after the <$2k automatic chrono market, but honestly they have that well covered with Eco-Drive and Precisionist/UHF 262kHz.
Also probably worth mentioning with Seiko is that they actually *do* have a 28,800 movement in the 6R lineup! They don't sell it to 3rd parties and only use it on a very, very few models mostly for JDM sale only. They're fully capable of producing a competitor to the 9015-- they
HAVE ONE RIGHT NOW, but refuse to sell it. They also have lots of experience with high-beat in general.