Bytecoin is, from what I hear, ~80% premined (as they waited 2 years to go public), has no gui and only has ring signatures.

Darkcoin is 0% premine, has a functional gui, has coin mixing and will implement IP obfuscation and ring signatures in V2.

They based their anonymity on using ring signatures that mask sender identities, you can look into the details by reading the CryptoNote paper. This has been recognize to be a good feature and will be implemented into Darkcoin in the near future. So from a competitive perspective Darkcoin will have both ring signatures and the current decentralized coin join implementation. On top of that, i2P ip obfuscation will also be added to Darkcoin, so competition is healthy as it keeps innovation going, but I would invest in the Darkcoin development team's ability to continue to improve on the solution and adapt to market changes.

Here is a quote from Evan on ring signatures.