The difference is just enough to keep your head from falling forward so you can sleep. The sad thing is the non reclining seats at the back of the plane and ahead of the exit row do not have this feature. When the guy in front of you uses his recline function you are double screwed if you are stuck in a non reclining seat.
The sardine effect is why I fly Delta, they are slightly better in the knee room department.
With my status on Delta, I almost always get at least Comfort+. However, on those rare times I end up in coach, I find that Delta has squeezed the seats closer than any other airline. Can barely move to even get in the seat. Occasionally, for a variety of reasons, I am on another airline. Was surprised to find that all Southwest rows are about like Delta Comfort+. Not a fan of Southwest for other reasons, but I do know Delta coach seating is among the worst.