Looks like the SparkFun WiFi Shield is a clone of the official Arduino shield so it will probably work with the Arduino library and examples. The adafruit shield uses a different chip and would require a different library.
Even if Adafruit CC3000 shield is using different chip and library, will it have the same features like the Arduino shield?
Adafruit is much cheaper than the other. I am planning to use Adafruit CC3000 shield to connect with my android phone.
SparkFun Xbee shield and Adafruit XBee adapter kit? Why is one called shield and the other called adapter? Any disadvantages in using the adapter on an arduino?
XBee Adapter kit again, is less than half the price of the SparkFun shield.
The "adapter" is not a Shield. It does not plug directly onto an Arduino board and establish reasonable connections. To use the adapter you will have to wire the adapter pins to Arduino pins.