Normal vs. slim is first a matter of glass vs. magnetic. Glass scales (like those pictured) are great at what they do, and offer good resolution for the price. The down side is that they can get dirty from all of the flying chips, oil, and coolant at your machine. Slim scales are now available in resolutions comparable to glass scales, but in a smaller, easier to mount package that is more resistant to chips and crud. Some installs do not allow much room to fit scales, so the extra space afforded by magnetic scales is a boon. Without a big cost or performance difference, today it makes the most sense to go with slim magnetic scales. The head units offer varying functionality and their capabilities are self-explanatory. There are a few common layouts based on whoever originally produced that style of head, i.e. Sony vs. Mitsutoyo. I prefer the Sony style (as a TI user) but people who like HP RPN calculators like Mitsutoyo better (get it?). It's preference at that point, because all heads will give you abs/incr, tool offsets, and bolt patterns, etc. so shop around for what you like. For a knee mill, it's nice to know what your Z axis options are, for instance.