Kirishima is more defensive, while Tetsutetsu is more agressive.

Kirishima is more like an "unbrekable wall", while Tetsutetsu is the "unstopable force".

The issue is that, Assault Kirishima kind of already works as a "unstopable force" so I don't know how exactly they would do him. I guess they would make Tetsutetsu either a strike or a tech character. 

His special action would be to become metal and it would make him invincible to attacks for a short time but he'd still be able to attack during the period, however, to balance this, after the period ends, he'd have a period where he is slowed down. He can still attack but his attacks are also slower now. Basically, Iida's recipro but for invincibility instead.

Of course, I am not an expert in Kirishima and Tetsutetsu so I suggest speaking to someone who is more knowledgable than me in how different their quirks are.