Tape allows creating a custom grid to measure distance between all set rules. It snaps to sides of elements (can be turned off) and font baselines, allows setting horizontal and vertical rules manually, generate them automatically and have a custom origin point.
Dimensions takes a screenshot of the page and uses that to create a contour map via edge detection that can then be used to measure actual page elements instead of HTML elements. For example, Dimensions measures against contours of letters in text, through transparent backgrounds, etc
Since Dimensions is based on color edge detection, color gradients and patterns tend to mess with the results. A simple workaround is to open Chrome's DevTools (or equivalent) and remove the offending DOM object.