Personally I don't really see a big difference between HP and DEF. Actually I am faced with the question of how I could make my Kalvalax more resistant. It tips over far too quickly. I now have the choice between (it's about the banner) more DEF or more HP: 

i

a) So which one would you choose, and why, please? 

b) Is it a relation between HP and DEF, like 10 DEF = 100 HP?

Thanks a lot for your advice.