the ct20b has a ceramic turbine (exhaust) wheel and a steel compressor wheel... the ct26 has a steel wheel on both sides. When the ct26 says "ceramic" on it, it is
in MOST cases refereing to the compressor wheel being ceramic COATED. Which increases the heat transfer rate on the compressor wheel. The ct20b will bolt directly on to the 2nd gen 3sgte manifold (though it will have one additional bolt hole that will be unused).
Here is a pic of a ct26 with coated compressor blades.
(http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=7954335233&category=33742&sspagename=WDVW)
The ct20b came stock on the 3rd gen. 3sgte. (ST205 Celica and 1994 and up MKII MR2... both JDM only)