Both methods have advantages. Bitcoin, for instance, is not pegged to any other currency. This introduces exchange volatility in the short term. But in the long term it will be much better off being independent. Pegged currencies are very stable, which promotes economic growth. China is a good example. But ultimately pegs become destructive and cannot be maintained.
At one point I was working on a proof-of-concept local currency backed by Bitcoin to promote stability: https://bitcointalk.org/index.php?topic=10970.0
(Ironically, the reverse of most currency pegging schemes)