Thanks for the answers everyone. More than my initial question - what I really am trying to understand is which type of anonymity DRK vs. MRO has more long term advantages? Whats the case for one or the other? Or will they simply be competing?
I hold both MRO and DRK and so have thought about this question somewhat carefully. DRK has the advantage of better awareness (particularly with its recent price increase), the familiarity and backend support of using a BTC-based client and associated services, and a large community of holders/miners. The developer, Evan Duffield, is quite visible and had made meaningful development progress. Some of the economic characteristics of the coin, including its relatively low inflation and the masternode system (which ties up coin) tend to encourage price increases. The principal concern for many regarding it is its unequal early distribution ("instamine") which you'll have to decide for yourself how much it bothers you. Another potential criticism is that Darksend is currently closed-source and unvetted, so we have no way of knowing if it will deliver on its promises. In fairness, Evan has stated it will go open source when complete, however.
MRO has the advantage of a truly novel and elegant implementation of anonymity. If a coin's value is determined purely by the anonymity if offers then CryptoNote beats CoinJoin (which underpins Darksend) hands-down. Even the originator of CoinJoin, gmaxwell, has stated as such. The CryptoNote developers, while anonymous, appear to be quite familiar with the academic cryptography literature and have used ring signatures in a clever manner to underpin their coin. Ring signatures are academically vetted and enjoy broad support within the cryptographic community (e.g. Adam Back, the cryptographer who invented hashcash for Bitcoin, tweeted his support). While I think Evan is a great developer amongst the sea of altcoins, he strikes me as more of a coder and less of a cryptographer. He, for example, has reversed himself and decided against using ring signatures. While this decision was ascribed to avoiding bloat in the blockchain, I think it's more likely because he realized implementing ring signatures in a BTC-based coin, particularly one with an already-established blockchain, would be extremely difficult.
The main limiting factor of MRO right now is that because it is not based on BTC, all of the underpinnings users have come to expect (GUI, pools, exchanges, etc.) have to be developed from scratch. This hurts the uptake of the coin and scares off less advanced users. It is possible that DRK could develop a lead in user uptake while these things get sorted out. With that said, I think the progress over one month has been very fast given that deficit; there is also a strong team of experienced developers. Some also criticize MRO as a "clone" of Bytecoin (the original CryptoNote currency marred by its own, bigger, "premine" controversy). While Bytecoin did indeed form the base of MRO, the MRO devs have done a lot to popularize CryptoNote and make it accessible to end users. MRO also has by far the largest net hash and exchange volume of the CryptoNote coins. As we saw with Tenebrix/Litecoin, sometimes being second with a fair launch is what makes the difference in making a coin stick in the long term. Also, the reason DRK is able to avoid having any "clones" currently is because of its aforementioned closed source; this is not really a strength per se as it is unlikely that people will tolerate a closed source coin in the long run, particularly when it comes to anonymity.
Anyway, I think both of these coins have a bright future. While the recent run-up of DRK clearly has elements of irrational exuberance, I do think that, so long as it delivers on its promises, it deserves to be beside Litecoin as one of the most valuable BTC derivatives. For a dark-horse long-term bet though I think MRO is the true second generation cryptocurrency.
After that promo it has 0% fees.