I have both attacked cities with epic walls and defended them, and the differences is that they can take much more damage (you will have to fire on them longer before they collaps), the siege towers looks like they are made completely out of metal (I havn't been able to confirm it, but it seems like they can take more damage too), there are more towers and the towers make more damage (they are the only thing execpt for siege weapons that can hurt urbans in testeudo from a range).

Roughtly the same as the difference between a regular and large stone wall.

I never noticed it in RTW, but in BI (and asumingly with the 1.3 patch, though it could have been there before) enemy siege towers depend on the type of your walls.

If you have normal stone walls the siege towers shoot arrows, no problem for most troops. If you have large stone walls however the blasted things seem to have multiple ballistae on board and standing on the section of the wall they are aproaching is extremely hazardous to your health.

So I never upgrade stone walls anymore.

Actually, the return for this is that epic stone wall towers also are equipped with their own ballistas. And they have unlimited ammo, as long as you own them.

They don't have unlimited ammo, it's just plenty. With Epic Stone Walls, you can't build ladders and I don't think you can sap. Towers also have Ballistae in them.

In campaign, the larger the walls, the longer a city can hold out when besieged.