If you get a chip from TTS, you won't have to worry about your chip not working if a dealer later updates your PCM with a new calibration.
The PCM in your vehicle has four pages of memory. Most chips on the market today (excluding TTS) replace one page of this memory, but depend on the other three pages of memory in your PCM staying the same. When the dealer updates your PCM with a new calibration, which often makes changes to the other pages your chip depends on, then your chip will no longer work and will need to be sent back to be updated to work with the new calibration.
The TTS chips replaces all four pages of the PCM's memory which is why don't have to worry about your chip not working after the dealer reflashes your PCM with a new calibration. The web site for TTS is www.ttspowersystems.com
I don't own a TTS chip, but I like the concept behind it very much and many here are very happy with the performance from their TTS chip. In my opinion, the guys at TTS have really done their homework and currently have the best chip solution on the market.
On the flip side of the coin is your vehicles warranty. Ford dealers have been known to void powertrain warranties if they suspect that a chip has been used which is something else to consider if you're at all worried about warranty issues. Good luck with whatever your decision is.