Are you looking brand new, as in 2021's?
I don't know about the reliability of the current EcoDiesel(3rd Gen) but the last gen(2nd Gen) was not great. I personally would not wanna own a 2nd gen and would be hesitant to own the 3rd gen until we see how they hold up.
The Ecodiesel will get better MPG for sure but, depending on the price of gasoline vs diesel in your area, it may come out in the wash in terms of cost to fill your tank. BTW, the Ecoboost motors are perfectly happy running on 87 octane. They will pick up some power on 91 or 93. As far as I know, the 400 hp/500 tq power rating on the 3.5L is using 87 octane.
Power wise, hands down the 3.5 Ecoboost. It will run circles around the ecodiesel, especially the current 2021 with 400hp/500 ft-lbs of torque. It's not even a contest. For heavy towing the 3.5L will pull more comfortably and you will likely never need all the power it has.
Payload should be in favor of the 3.5L. The Fords are already a little better, and with the added weight of the diesel the Ram will be at a disadvantage if the two trucks are equally optioned.
Historically the rams have ridden better. I don't know if that has changed with the new F150, but I can tell you that I have rented 4-5 Ram 1500's of the last few years for work trips and they ride very well. Way better than my stock F150, and probably slightly better than my F150 with Bilstein 5100's.
The 3.5 Ecoboost has had issues with the cam phasers. I don't know if they have been updated with the release of the new 2021 F150. I had heard that somtime in 2019 or 2020 they had come out with an updated design so it may not be an issue anymore.
If you are not towing or using the truck for heavy work, I would be looking at the 2.7L Ecoboost all day. It gets better MPG's than the 3.5, but still performs extremely well and has so far been pretty dang reliable from what I can see on the forums. The 2.7 is a very solid motor and is really all that most people need.