Although popular in California and New York, Pinkberry is a fairly new concept here in the Midwest, and it took the city by storm. At only $2.50 ($3.95 with toppings) it's a great way to satisfy a sweet tooth. Although the texture of the frozen yogurt can be a little runny after a while, the pleasant and serene atmosphere makes its few flaws hard to notice.
By contrast, Red Mango has been around the Chicagoland area since 2008 and quickly became popular. Like Pinkberry, a small cup of froyo is only $2.50 (with extra charges for toppings) and with the huge variety of dry and fruit toppings, you can't go wrong with it. The laid back feel to the store is always inviting and it somehow always makes me want to come back for more.
Although both are delicious and fairly cheap, my top choice for a frozen treat would definitely be Red Mango. Its rich flavor, yummy toppings, and the truly frozen-till-you-eat-it texture makes me happy to be alive. Whatever your preference may be, have fun, eat up, and enjoy!