Note that while I've done a good bit of modding for Minecraft, I'm very new to Minetest, and have only started experimenting with it in the last couple of days. The information in the Minetest half of this post is what I've gathered from reading about Minetest and its API, and from playing a few games in Minetest, and may not be fully accurate. If you notice any inaccuracies, let me know and I'll edit this.
Minecraft:
- Already a fully fleshed out game. Great if you want to do things that Minecraft does, but can become more difficult (or straight up impractical) when attempting to go too far beyond that.
- Has a very in-depth unofficial API (Forge) (And maybe also Fabric now, though I can't speak on that personally as I've only used Forge)
- (As a sort of sub-note, you can just straight up directly render things with OpenGL in a Minecraft mod, which is nice, though admittedly a bit niche. As far as I know, Minetest doesn't really have an equivalent to this.)
- Breaks mods a lot between major updates, and often necessitates that modders relearn large parts of the modding process
- Not FOSS, which makes modding a bit legally gray, though Mojang/Microsoft have thus far been mostly content to leave it be
- Has a large player base. This can potentially result in more players playing your mod, but due to the vast number of mods, there's also a greater chance of it getting overlooked
Minetest:
- An engine for voxel games. Okay if you want to mod a Minecraft-like game, but its real potential is in its ability to make games that aren't Minecraft.
- Has an official API for making games, and games (and mods) can provide their own APIs as well.
- Doesn't break much between major updates, and the API stays generally the same
- FOSS, which makes the legality of modding very straightforward
- Does not currently have a large player base. This means that there are less people to play games/mods, but a well-designed game/mod will also likely stand out more.