When comparing KVIrc vs Nettalk, the Slant community recommends KVIrc for most people. In the question“What are the best IRC clients for Windows?” KVIrc is ranked 5th while Nettalk is ranked 11th.
Ranked in these QuestionsQuestion Ranking
Pro Plenty of customization options
KVIrc does not need to be installed. It can be kept on a flash drive and run from there.
Pro Still active developement
Despite whats been said, development has been continuing at a steady pace on version 4.3.1 and more recently 4.3.2.
Pro Extensive documentation available
Help files and online resources show all of the feature and scripting capabilities.
Pro Can overlay incoming messages
Nettalk can be set up to overlay incoming messages. Especially useful for games.
Pro Encryption support
Pro Multi-lingual interface
Nettalk is available in English, Spanish, Dutch, Russian, Chinese, Hungarian and German.
Pro Built-in spellcheck
Pro Functionality can be extended via plugins
Pro Free and open source
Con No longer developed
Development ceased some time ago (2012)
Con German language
Is primarily a German language client, so the spell check, etc etc is in German.