I think we're talking about the same formation.For me, the 3421 we played for most of the season we finished 2nd to Chelsea is the best we have played. We haven't looked as strong since that season. Dele in particular has never looked so good, he was magnificent that season (18 league goals, 22 in all comps). That system was perfect for him.
The flexibility of Dier, Alli and Eriksen meant that the back 2 could become a 3 whenever required and Alli/Eriksen could move between midfield and further forward depending on where the numbers were needed.
If you have 3 players as flexible and intelligent as that, then the two formations are very fluid.