GMS vs WIFI

I have a project where I am sending data to the internet. I am using the Mega 2560 and the Arduino WIFI shield. However, I will put this project in a box and the internet connection maybe weak. I was looking into a shield with strong signals and came across this link. Recommendations For You - DealeXtreme

The wifi I got is from this link http://arduino.cc/en/Main/ArduinoWiFiShield

I know they do different things and are best suited for they own application and comparing would be hard. I am just trying to find out for my application, where I am merely sending data to the internet and receiving data from the internet on a txt file on a server. Which device would be better considering I maybe 100 to 300 feet away from a modem or router? What ratings do I look for when determining these performance characteristics? Thanks.

arios85:
I will put this project in a box a
Which device would be better considering I maybe 100 to 300 feet away from a modem or router?

OK, it goes in the box but it's only a box and GSM and WiFI not entirely dependent on line of sight, but 300 ft could be stretching the friendship for WiFi anyway. This to the point where the WiFi receiver also demands consideration.
It can't be hard to prove the range of a WiFi but, if you don't have a chance to experiment, I think you should go for GSM

Thank you for your advice. I will get the GSM and do some experiments with the signal strength on both devices and see which one has a stronger signal with respect to my location and situation. Thanks for your help and will report back on this thread in about a month or so.