You can easily get parts for either if they are needed with great customer service. There is no question that the Hornady is a more heavy duty and robust reloader. I have used my 366 for over 30 years and it's still making shells like it did when I got it. The MEC is made out of plastic and sheet metal stampings that total so many pieces I doubt their sales people could tell you exactly how many pieces are required to assemble one. It's mind boggling to me how flimsy they are and everything is supported on one center post that flexes with every stroke. The shot and powder bottles are a joke and the support you can buy to help support them is another piece of plastic and is considered an option you pay more for. If their design is good why the hell would you need to buy an additional support from them.
366 if for no other reason than that the 366 has positive movement of the charge bar both ways, the Mec has positive one way and return spring the other, you will get hang ups with the Mec system causing some dud shells
There are arguments about the robustness of the 366 but I have seen Mec with hundreds of thousands of rounds loaded and they still work, reliability is great on both
If the Mec had both way positive on the charge bar I would go with Mec
By the way I own 2 Hydra Mec and 2 366 so my comment is real life experience
Both are good units, but the 9000 has automatic shutoff for the powder and shot, better (collet) resizing, one less move for case resizing, and cases are removable at every station. Also, the automatic/independent indexing (gas piston) on the mec is superior.
If you don't mind resizing/depriming all of the hulls a separate operation before reloading, the 366 becomes a much smoother machine. Get the new style 366 spring-type primer seating ram. The 366 also prefers hulls in "good" condition, due to the swinging wad guide thinner petals. The 366 has a better primer feed system, and the entire machine is much more stout.
Parts availability is good for both machines (internet), but it seems more shops stock the mec items, as it is a more popular, better selling line.
They all work as advertised once you learn the quirks of each loader. Yes, they all have there own little quirks. They also all require human intervention to keep the machine running smoothly as do all machines of any type. I suppose they all have their individual advantages as listed above. I happen to be a fan of the Pacfic/Hornady 366 but I also own two MEC's as well. In the end, you will seldom be able tell which shells came from what loader when the machines are adjusted correctly.
My Pacific 366 adjusted correctly. Perhaps I should make a video of when its not adjsuted correctly but it would need to be edited for foul language.
Having had 1 of each-- the 366. Mec is long gone. For $$$$ invested the 366 is probably the best machine on the market. After 30+ years I finally needed a part (my own fault) for my old Pacific, called Hornady to see if that part would be the same 30 years later and explain (admit) that it was my fault, they said yes, 2 days later I had the part NO CHARGE. Hard to beat customer service like that. Ross Puls
EDIT---- Just watch right here on T.S. for a couple months and see which comes up for sale (used) most often, then you will know who keeps which machine and which ones are always looking for new homes. Should tell you what you want to know.
Over many years I have owned Hornady 366, PW800+, Dillon 9000, Mec 650JR, Mec 9000G and Mec 9000H. I liked my 366's, really liked the primer feed, never a failure. Had the PW800+ for a year and couldn't wait to get rid of it, just didn't like the feel,leaked powder, primer drop was a pain.
The Mec 9000G a really good loader but liked the Dillon 9000 for its smoothness and leverage. Hands down the Dillon is the best to me for a hand reloader.
I own two Mec 9000H's with use of one pump. The 9000H I will never get rid of. They load all my singles and first shot of doubles load. If I didn't have the Dillon I would load with confidence my handicap loads. The 9000H never miss a beat unless I screw up. Easy to maintain and parts inexpensive.
If I was buying one new loader now I would get the Mec 9000 either with Automate or with Hydraulic System. If I didn't have the money I would buy the Mec9000G and add the Automate later (Check first but I believe you can still add Automate later).
It's the same old question Fords or Chevys..Drive whichever you prefer. Load on whichever YOU prefer, they both produce great, shootable shells if you do you part. Buy the one you can afford and work it to death..might take thirty years or so to do it. My Mec hydro 650 is about thirty years old and still going strong. Its only downer is it doesn't have a sizer so I have a Mec sizer mounted right next to it on my bench. Size about 50 and then load them. Gives a chance to look the empties over first.
Maybe 12 ga 366's are different but I have 366's in 20, 28 and 410 and they sit in the corner after numerous tries. Don't get me wrong as they work but I have Mec 8567's that I much rather use. I do use a 9000 for 12 ga and love it too. The only plus that I know of with the 366 is free replacement parts. All but one time I called Hornady and they sent me the parts free of charge. I rarely need parts for the Mec's anyway except for the resizer on a 28 ga loader recently and it was expensive. I have nine Mec's and if I had to purchase many parts I would go with the Hornady. Jackie B.