iPhone Video Capacity?

Am considering getting a new Video Camera for simple stuff. Looking at the Sony Bloggie Duo - says it holds 2 hours of Hi-Def video on it's internal 4Gb Flash Memory. However, I also plan on getting an iPhone (waiting for 5 at this point). In regards to video capacity, how much space does Hi-Res video take up on the current 4s iPhone Model? ie. How many Gb's/Hour of video?

Also, everything I have found searching says it takes many more Gb's to hold 2 hours of video on an iPhone than on the Sony Bloggie claims - why is this? Different compression?

Just wondering if I should just go with the iPhone and forget about getting the Sony.

Thanks,

Steve