What is your budget, and how detailed is your analysis of the data?
Like others have mentioned, you'll want to get an external GPS. I started using Harry's Laptimer with only the iPhone (and OBD-II), but the results were wildly different and not pinpoint accurate on GPS. You'll want a GPS with 10hz or above.
I just bought a VBOX Sport and I highly recommend it. This is top of the line equipment. It refreshes 20 times per second (20 Hz) and is very accurate. The unit will store the data on an included SD card, plus if you pair it to your iphone, you can use their laptimer app to see it live. Also included is their own desktop (windows) software for data analysis which is pretty powerful. Their files work with racerender if you want to overlay on a video too.
VBOX Sport - Racelogic USA Store