The 3.5L Ecoboost engine is just a twin turbo set up. A diesel engine will also be turbocharged. Complexity wise, diesel engines tend to be more complex and also have additional emissions equipment for particulates (ie - DEF fluid, etc). The 3.5L Ecoboosts have proven to be reliable engines since they were introduced in 2010 (Second generation of these engines in 2017). The main concern with Generation 1 was timing chain stretch - this problem was corrected with a hardware material change and the move to a two-chain system in Generation 2.
You’re going to get better fuel economy with a diesel, yes.
Your towing fuel economy will also be significantly different than what the window sticker says.
I just recently upgraded my 2011 F150 5.0L to a 2017 F150 with the 3.5L Ecoboost. Its wonderful and just easily hauled my 2H bumperpull with two horses last weekend. Zero issues - had plenty of power and torque to get up to speed on the highway. Honestly felt lighter than with my 2011 V8. The 10 speed transmission on the 2017+ models does take a bit to get used to. Really easy to get into the boost range, and therefore eat gas. I’m averaging about 21 mpg daily driving (mostly city driving with a 20 min highway trip to the barn). Towing with two horses last weekend, I averaged about 12 mpg. (I have the upgraded 36 gallon tank which allowed me tow 4 hours on about 1/2 a tank of gas). With the change in 2015 to an aluminum body, the payload capacity on these trucks went up significantly (which is the main reason I upgraded). The 2015+ F150s are ~1,000 lighter than the previous generations. My 2017 has a towing capacity of >10,000 lbs and payload ~2,000 lbs with the max tow package, supercrew/FX4. Be sure to double check all the specifications before purchase, of course. On any truck, towing ratings vary by cab size, engine, gear ratio, etc.
Happy to answer any other questions you may have. 