The simple fact, though, is that your rage can easily become 100+ as a warrior without taking it on level up at all, or at least, only when the other option is even worse. Apart from the +30 from that one skill, you usually find several shrines to increase it as well as several items and so on.
The argument here is not about utility of high max rage (I'm not disputing that it's useful, and it's one of the things I make sure that my warrior has. 50% kill with soul drain for 70 rage doable every turn is devastating, especially since you can do it on the first round and then timeback your most hurt troop the next round), it is about whether the choice between attack or rage on a level up is better. The answer is attack by a clear mile, especially because the OP said he was playing hard. The higher the difficulty, the more you have to rely on your troops to do your damage since there are more enemy troops and no increase in your ability to cast spell or use rage. Attack has a direct effect on that damage whereas the max rage will slightly increase chance of crit and help you to use rage abilities that are less useful a very small amount more often.
So to sum up, if you are a warrior, pick the attack over rage, but pick rage over most other things (except leadership, naturally). General idea for level choices for warriors:
Before level 14: Attack>rage>def>mana>int>leadership
On and after level 14 leadership>attack>def>rage>mana>int
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