Immibis's Microblocks lets you put covers (and other shapes) on things, and that's it.
Forge Multipart is a full-blown solution for putting multiple things in a block.
Immibis's Microblocks is more compatible and less intrusive. To make a mod work with FMP, the author has to rewrite significant parts of it; if they don't want it to require FMP as well, then they need to keep both the FMP and the non-FMP version. And if you ever decide to remove it, you'll only lose the microblocks.
However, Forge Multipart does more; it lets you put several things in the same block, not just one "main thing" plus some microblocks. (E.g. You can put energy conduits and red alloy wire in the same block, or several torches and levers).