Honestly, unless you do allot I like the Ebay one. It clamps on the blade (make sure it's tight or drill and thread holes in the saw blade bar thing)
and rides along a 2x4 that is screwed to the log. There is a little video on Youtube about the different thigns you can do with it. Some guy built a whole log cabin with that.
for efficiency or doing allot I'd look at the Logosol.
I have a Stihl Farm Boss with an extended blade. I changed out the sprocket so I can run a longer narrower blade. You'll need to get a "ripping chain" which is reground to cut most efficiently along the grain of the tree. Most chainsaw blades are designed to be great across the grain. I milled 5 large oaks with it into a huge pile of lumber for a former employer. It took a few days.
It's not as efficient as a bigh porta band saw mill..... but it's not $25,000 either
There is a mial order arborist supply website. name eludes me right now but they have great ripping chains. Really efficient. I can cut down the center of a 2' diameter log almost. But actualy you wouldn't realy do that. Take a side shaving off to create a flat surface. Move the guide bar and then the 2nd cut creates a straight corner. After that you can cut lumber or staves what ever. If the log is thicker it' ok you could just rip a deep trench then usse splitting wedges to open it up. In some cases the wedges may be more efficient. I breathe allot of exhaust when I use mine. As an arborist I'd be fine but for the mill the saw rides on it's side , which blows the exhaust right in your face.