Re: which keeps liquids colder, a plastic or glass cup?

It depends on the specifics of the glass and the plastic. Both come in a 
variety of types.

Plastics are more likely in a practical sense to keep things cooler because 
they can be more easily made into foamed forms (the classic styrofoam cup is an 
example). The trapped air in the foam is a good thermal insulator, and thus 
less heat is likely to get through the cup walls to the contents.  However, 
special types of glass foams (known as xerogels) can be made up to 99% air by 
volume, and these are extremely good thermal insulators. The tiles that cover 
the space shuttle are the best known example of that, but I doubt you'll find  
a cup made from such xerogels (they are somewhat expensive to make).

To decide which of cups available to you are best, an experiment is easy to do. 
All you need is some cold water from the refrigerator, a thermometer, and the 
cups. Fill the cups, including one extra one, with equal amounts of the cold 
water. Measure the initial temperature in the extra cup with the thermometer 
(and keep the thermometer in that cup except when taking measurements...this 
will minimize any heat transfer due to thermometer insertion). Now take 
temperature measurements at regular intervals of the liquid in each cup. Plot 
the results.