Playing Defense

You should try playing around with your D-man's height and weight. But you'll probably sacrifice some speed and agility for checking power.

if you're referring to EASHL, i haven't noticed any difference. If anything, I think they made the d over powered with spam poke checks and being able to send someone flying by bumping them after they've already past you... the only thing i've seen that
has some effect is when the people you try to hit have their balance and strength maxed out

Question please, Is there any way to play "BOTH" offense and Defense while playing NFL 15? It lets me play offense no problem, but then it simulated all kicking and defensive plays.

Advice? Learn to play defense, it's the most important aspect of the game and tough to master since you have fewer tools in your arsenal to play good defense. Play a lot of career mode and gradually work up the difficulties. Don't hold A as a crutch to defend.
Work on positioning your defense, timing your tackles and your overall team tactics.

The thing I like about hardpoint in TF is that it rewards you for offense and defense. I've played offense and defensive and the XP points are good for either playstyle.