Comparing Monero and Zerocoin, here is a post I made in the Anoncoin thread.
Ok, as an example let's consider ring signatures. A solution that provides cryptographically guaranteed anonymity without the risk of keys that can create unlimited coins. Who in their right mind would prefer Zerocoin/Zerocash? Let's use a gas tank in a car as an analogy. Would you prefer a gas tank that has a hole, which has a metal plate welded over it, or one without any hole to begin with? The metal plate should theoretically prevent leaks, but who would choose it over a tank with no hole to begin with?