There are three main items to consider when purchasing an insulated cup.
First, you must consider price. Price is the leading characteristic that drives any retail product, but it is especially important when choosing an insulated cup because there is such a wide discrepancy in prices. Insulated cups can range from $4.00 to $80.00, with little distinction in some cases. Like other retail consumer discretionary items, however, insulated cups do suffer from name brand associations with level of quality. And as you know from experience, many purchase their particular product based on brand alone; we do this particularly more when it comes to items that others will see on us or with us. Think watches, cars, clothing, etc. Keep in mind that the smart shopper considers not only quality but also price.
The second thing to consider is quality. You definitely should look for an insulated cup that has high quality plastic and or other materials. Be wary of cups that have very weak seals between the inner and outer layers of the insulated cup. Color can make you more satisfied about your insulated cups but ultimately, quality should be the most important factor because you certainly don't want to have to reinvest your money into the cups.
Also, you may want to consider is functionality. Don't buy an insulated cup that has a side handle on it if your car's cup holder won't accommodate it.