I ALWAYS say it, and always will. Narrow formations are always better on FM.
I find it depends on the level of your team compared to those you're playing. With a dominant team, a wide formation usually works quite well regardless (obviously you've got more space to move the ball around and stretch your lower-ability opposition).
With a mid to bottom-table club, a narrow formation generally works, keeps things tight and limits better teams from carving you open, making them rely on deep crosses more.
I've found defensive midfielders much, much more effective in FM14 than before. Previously, if I was playing 4-3-3, I've been happy to have 2 sitting in the middle, an attacking playmaker with 2 wingers and a advanced forward. Now, I'm having more joy bringing a guy back into DM and using 2 inside forwards with a deep-lying forward up front. With my team fighting to get into Champions League places, I don't find much difference between 4-3-3 and 4-1-2-1-2, each depending on the opposition's formation (though I find if they're especially narrow, I have to be too).
One old rule I've used forever, which goes against what many of the popular plug-and-play formations use, is when using an attacking formation, I have a deep defensive line to create more space and draw the opposition out. When I'm looking to hold on to a result or playing a bigger team, I'll have a high defensive line, tightening up the space between the defense and midfield, looking to limit their flair players a bit more.