Best way of seeing how they're different is to play against each of them. M28 is a bit harder, runs faster (relevant if you have a bad CPU or play on large maps with lots of players), is more compatible with mods, and has a range of customisation options.
M27 is a bit more 'player-like' in how it manages units (M28 coordinates its units as a team, whereas M27 does it on a per-player basis for the most part), and is more heavily impacted by intel (if it cant see a unit it doesnt realise it's there; whereas for M28 it remembers if it saw that unit a few seconds ago even if it loses intel). It also has a few strategies it still does better than M28 (such as rushing a strategic bomber, or building a firebase on a map like astro craters).
