TARK is held every other year. Next time in 2011 likely in Holland. Krzysztof Apt is the sole PC chair.
EC is held every year. In 2010 at Harvard, June 7-11 overlapping STOC and Complexity, both also in Cambridge. A mini-FCRC a year before all three are in San Jose for the real FCRC. Moshe Tennenholtz and Chris Dellarocas are PC chairs. Yiling Chen and Jenn Wortman are doing local arrangements. David Parkes is general chair.
TARK drew about 50 attendees, EC is drawing about 175.
TARK talks were held in a small classroom in the old GSB building. EC talks are being held in a large ballroom in the Alumni Center. The atmosphere encouraged questions after TARK talks and discouraged them at EC.
TARK has no industry support and few industry participants. EC gets money from several companies and has many people here from Google, Yahoo, Microsoft and others. Yahoo bought us lunch yesterday at EC, tonight's banquet is at the Googleplex.
EC has a new general chair every year. TARK has had one general chair: Joe Halpern.
TARK had traditional printed proceedings which are now available on the ACM DL though not an ACM sponsored conference. EC, sponsored by ACM SIGecom, has CD only proceedings not yet available on the DL.
TARK focuses on models, most papers give logical models of knowledge-related concepts and discuss theorems but rarely give proofs. EC has many new models and mechanisms too but more of an emphasis on proofs in the theoretical papers.
EC has a continual identity crisis on broadness and applicability. TARK did change its name a few years ago (from Theoretical Aspects of Reasoning about Knowledge to Theoretical Aspects of Rationality and Knowledge) but is quite happy with what it is.