Coolant temp sensor vs. fuel temp sensor

[Jimwsea]

What is the difference between the coolant temp sensor and the fuel temp sensor in terms of how they work and what problems they cause when faulty?

I've read many threads related to a bad coolant temp sensor hurting fuel efficiency. I've also read threads on how to replace the fuel temp sensor, but I must have missed discussions on what mpg improvements that made.

According to vag-com, my CTS was reading 15-20 degrees high, so supposedly it was overfueling the car. I changed it yesterday. It hasn't left the driveway since, but I will be I'll be tracking mpg and noting any changes in power. I'm just wondering if I should go ahead and change the FTS (no codes on the FTS) after seeing pictures of how rusty and gunky they can get.