So I went through this decision awhile back and I ended up with a 2011 4x4 lariat supercrew w 3.5 EcoBoost. I've owned it about a month and a half. Its currently in the shop getting its 8 foot long timing chain and all the cam phasers replaced at 100k miles. This problem affects vehicles through 2015.
The ecoboost has amazing torque, burns regular gas just fine. The transmission is great and the factory coolers handle towing my 8000lb boat without heating up more than 10F over the normal unloaded temp.
The Ecoboost does have a few common problems that you will read about if you search the internet.
1) Loss of power in heavy rain/high humidity conditions. This is an issue with condensation in the intercooler. I have driven it in heavy rain and never had this problem.
2) Timing chain rattle - In the higher mileage trucks there is a problem with the timing chain tensioners and cam phasers, resulting in a loud rattle at startup when the truck has been sitting for over 8 hours. Mine had this problem. Ford has released updated parts and this is covered under the extended powertrain warranty. I did not buy new, but I think the extended warranty is around 1k extra when buying and covers you to 120k miles.
3) Intake valve carbon buildup. This is a problem with any direct injection vehicle. The fuel is injected directly into the cylinder and so the only thing flowing over the intake valves is air and crankcase fumes from the PCV. Over time the gases from the PCV system cause the intake valves to get gunked up. The 3.5 ecoboost does not seem to be experiencing these issues as bad as other direct injection vehicles. At 100k I have a very slight rough idle and i'm getting 2 mpg worse than the low mileage loaner I have from Ford (same truck, engine, drivetrain). $600 or so to have an independent shop clean the valves (ford does not recognize the issue) or you can DIY in a half day. The valves are fairly easy to get to and you only need a shop vac and some long shafted wire brushes + cordless drill.
In order to prevent the carbon buildup, many owners are cleaning the valves and then installing a "catch can" which basically filters the crankcase gases before they go back into the intake.
I knew about all of these issues before I purchased the truck. I didn't have a chance to cold start the vehicle to check for that rattle, but the warranty I got with the truck covered it. In the end, I decided the extra power for towing the boat was worth the potential extra maintenance over the v8. Also its fun to drive with the extra torque. It makes the full 420 ft lbs of torque at 2500rpm and it makes 90% of that at 1700rpm. You can set the cruise control at 75mph and go up a steep hill, never downshifts.
