I tried looking up definitions for the two words and found the following:
Aggression:feelings of anger or antipathy resulting in hostile or violent behaviour;
synonyms: hostility, aggressiveness, belligerence, bellicosity, antagonism, truculence
readiness to attack or confront: forcefulness.
synonyms: confidence, self-confidence, boldness, audacity, self-assertion, assertion, assertiveness, self-assertiveness, determination, forcefulness, vigour, energy, dynamism, zeal
Intensity: the quality of being intense
synonyms: strength, power, powerfulness, potency, vigour, force, forcefulness
So, from these definitions, it would seem that aggression is mostly about looking for the fight, whereas intensity is more about focused energy. For examples of aggressive players (and this is just my opinion), I'd cite the likes of Farrell, Brown, Hartley, Liam Williams, Mike Phillips, Morgan Parra, Eben Etzebeth, Jim Hamilton, Cian Healy and Bakkies Botha. For intensity I'd pick out Pocock, Carter, Faletau, O'Driscoll, Alex Dunbar, Jonny Gray, Stephen Moore, Sergio Parisse, Nakarawa, Rob Kearney and Ma'a Nonu. It's not an exhaustive list, but I hope you can see what I'm driving at.