Question

What is more important for a Wall, HP or Defences?

Answer 1

The value for the two is equal.

You need HP to last longer and you need Defense to take less damage from hits. One supports the other. The usual Wall sets are 252 HP/ 252 SDef/ 4 Def or 252 HP/ 252 Def/ 4 SDef so you can clearly see they are both important assets to a wall.

Comments

I'm to sloww... :P

---

I agree :)

Answer 2

Both. When the damage get calculated, you prevent most damage by having balance between HP and defense stats. So, if you want a Pokemon - with high HP stat -, to be your wall, then it's best to invest EV's in Def/SDef.
My source is pure experience :)
Hope this helped!