I had the 1.4 for five years and a Mk6 for three years before that, which had an earlier version of the same engine. It’s quite solid, and I never encountered any issues with it.
The 1.6 is definitely more powerful, but when I test drove one, I noticed it felt a bit lacking in power at lower revs while just cruising around.
I’ve owned a 2012 Fiesta 1.6 Turbodiesel for the past two years and have driven it all over Europe. It’s surprisingly comfortable and economical, consistently achieving 4-4.5 liters/100 km. It hasn’t burned a drop of oil in 25,000 km (and I’ve made sure to change it regularly).