Initial benchmarks with Parsoid and RESTBase indicate a 10-15% speedup from the newer v8 version shipped in iojs. It also efficiently supports modern ES6 language features like native promises, generators, collections, classes, arrow functions or template expressions. See https://github.com/lukehoban/es6features for a good overview. Especially generators can potentially make our async code quite a bit more readable, and are on track to become even prettier with async and await.
As soon as there are decent Debian packages for iojs we should IMHO seriously consider upgrading our nodejs stack. We are already routinely testing Parsoid and RESTBase against iojs via travis, and have not seen any issues.
Regarding nodejs 0.12 vs. iojs: just from looking at https://github.com/iojs/io.js/graphs/contributors vs. https://github.com/joyent/node/graphs/contributors I think it's fair to say that the momentum has shifted significantly towards iojs. Node 0.12 ships with versions of v8 (3.28) and libuv that are out of date / no longer supported by upstream. The platforms remain highly compatible (important features like generators are also available in 0.12, albeit slower), so the risk of choosing the faster implementation while the longer-term node roadmap sorts itself out seems to be limited.
The iojs Debian packages are called 'iojs', and provide nodejs if installed. This means that we can offer iojs along with Node, which lets us upgrade clusters individually, with nothing changing by default.