I have very little experience with anything besides my Nakajima 280L so that is all I will comment on.
I purchased my Nakajima 280L about a year and a half ago and I am quite happy with it's performance. It still punches through 20+ ounces of Vegtan and Latigo leather with ease. The only problem I had was that the (feed Dog?) would leave marks on the bottom of the leather. You can buy smooth feed dogs to eliminate this problem, but I was in a hurry so I simply removed the feed dog, filled in the grooves with a small amount (Pea sized) of JB weld then lightly sanding it smooth. I ordered another feed dog in case I needed to switch back but so far have never needed to and the system is running like a champ. From my understanding, the 280 was built like a tank and my experience supports that. I have purchased other feet and accessories for it and they are easy to find.
The only quirk is that it does require hand oiling as it does not auto lubricate but that takes less then a minute and I only do it at the start of the day. And even that may be more often then is needed.
At the time I purchased it I could find no negative information about it and those reports I did find gave it good to great reviews. My original post on it can be found here as well as a link to a manual.
http://leatherworker.net/forum/index.php?showtopic=32863&hl=+nakajima%20+280l#entry204504
As for the Yamata I have never used one so I can not say anything good or bad about it.