ComputerCraft
For those that don't like images: the wiki has moved to a new place, http://ocdoc.cil.li/.
This wiki will no longer be updated.
Not as much as it may seem. I personally do feel that while yes, the mods share a lot of core concepts, the mods are still very different. And I prefer to stick with "different" over claiming one is better than the other. They target different people, who like to play their game in different ways. ComputerCraft invites you to experiment and gives you all the power you need to do whatever you possibly could with it. OpenComputers tries to regulate that power to fit and integrate better with other high tech mods, in the hopes of making it feel less out of place in a survival environment. The mods have different goals. ComputerCraft is a Swiss Army Knife. You get one block that can do anything. OpenComputers is a toolbox. You have to pick the right tool for the job - computers are modular. At least that is how I see it. I do realize however that some of the differences may not be apparent from the first impression, in particular when you're used to ComputerCraft. And if you cannot be conviced otherwise even after playing the mod for a bit, just think of it as healthy competition ;-)
- Fully persistent computers. ComputerCraft computers have to reboot whenever the chunk they're in is reloaded - for example after all players went out of range or you quit your single player game. This mod's computers don't. They just continue where they left off. This is possible because this mod uses the native Lua library with a low level add-on that allows full state persistence, including the call stacks of active coroutines. If that's too tech-speak for you, just know the programs running on this mod's computers will keep doing what you told them to do until you tell them otherwise (or they run out of power).
- Limited resources. While ComputerCraft also limits a computer's disk space this mod additionally limits its memory. This adds a new level of depth to programming your computers, and at the same time makes it a bit harder for malicious - or just careless - users to run your server into the ground by allocating a ridiculous amount of RAM. The available memory sizes are fully configurable.
- Open source. I prefer public auditing: I feel a mod that allows users to run arbitrary code on your server should allow you to review its code. And hopefully this'll make it less likely for security issues to slip by.
- No thread spam. Instead of having to create one thread per coroutine (three per ComputerCraft Computer per default, I think), OpenComputers uses a thread pool of configurable size.
- Computers aren't magic all-in-one blocks. You'll have to put them together from different parts to make them do what you want them to.
- Things are not dirt cheap. Everything has its price, and higher tier, more powerful components cost a lot more than the basic stuff. And even the basic stuff costs quite a bit more than a ComputerCraft computer.
- Computers' flexibility comes at a price: energy. Keep in mind that this is fully configurable, though!
- Way different equipment system for robots vs. turtles. This allows re-configuring robots later on, which makes sense, since you'll want to take good care of your robots (hint: they gain experience).
- Lua 5.2, which allows a lot more freedom by enabling you to yield from almost anywhere. Also, no more stupid getfenv/setfenv craziness.
- Native libraries. To allow persisting callstacks we have to use native Lua libraries. This means availability for hosting is limited to platforms where those libraries are available. At this time that is: Windows (32+64 bit), Linux (32+64 bit) and Mac OS (32+64 bit).
- Native libraries. If something crashes in the native library the game will immediately crash. The world cannot be "emergency saved", so you'll experience a roll-back to the last save. There may not even be an overly helpful crashlog. This is rather unlikely to happen, though. Really. I promise. I'll wager it's more likely OpenGL will crash because your driver is too old.