Both are important IMO. It's a good idea that your Def/SpD stats are high enough to take a good hit, but high HP helps as well. In my head I like to keep Def/SpD on a higher level than HP, unless a Pokemon wouldn't benefit from it, like Scizor for example. His defenses are IMO helpless, so I boost his HP EVs to keep him alive longer --- Feraligatr is my favorite pokemon. 3DS FC: 0216-2213-1742
It's really kind of a case-by-case basis, but GENERALLY you can look at it this way. If you max Defense out, you'll take physical hits better than if you maxed out HP, but you'll take special hits far worse. If you max out special defense, it's the same. If you max out HP, you will survive both types of attacks a little better, but not as well as maxing the respective defense stat.
On a defensive pokemon, the usual practice is to max HP, and max either its better defense to take those especially well, or to max your worse defense so that it becomes less of a liability. --- 3DS Friend Code: 5455-9401-7637 MH3U: Souretsu