[cowtools]

One is a chaumian cash system, the other is cryptocurrency with sender-obfuscation by ring-signatures.

The technical differences are too many for me to enumerate here. They are completely different technologies that make different efficiency-privacy-decentralization tradeoffs.

Functionally, Taler (and other forms of chaumian cash) are payment systems and not currencies. It requires a bank to implement it, in this sense it is less de-centralized, but likely much more efficient as there is no need for network consensus (because it depends on a central entity's blind signatures, etc. anyway). It has different (possibly better) privacy charichteristics than monero. They are both free and open source projects.