Well, or spoilers of course, spoilers are rarely ferrous.
In a jist, yes, those are the main differences between the two meters. There is a LOT more to it than that though, folks who have researched in depth paint meters will have a lot more to say. Basically the Highline is the cheapest paint meter that, well, works. There's a bigger margin of error on accuracy, and it can't read on non-ferrous surfaces, but it's cheap. The Positest is more accurate, and it can read on more surfaces, but it's much more expensive.
Then you get into the really expensive meters that can tell you the readings on each layer, such as how much base coat there is vs clear coat. IMO, these are the real meters, and what we would all want. A total reading of "122 microns" doesn't help me if the clear is only 1 micron thick... I'd personally want to know the depth of every single layer... But again, it all comes down to cost and what you're willing to spend.