The lore says that Lion el'Jonson and Leman Russ are evenly matched fighters. Russ is a bit stronger and Jonson is a bit faster. Russ bore a grudge against Jonson because the First Legion, and the Lion specifically, could claim more victories than himself. Both were raised in the wilds of Death Worlds and had feral streaks. Most people forget that the Lion was mostly plagued by psychological/sociological flaws stemming from his extreme isolation on Caliban which made him poor at understanding human nature and humor. He was paranoid and extremely prideful. Russ was also prideful but had a unique sense of humor having spent more time amongst the human population and being raised in a family. This led to their confrontation on Dulan's World and an honor duel waged over the killing of the Tyrant Durath.
I respect Leman Russ but I've never been a true fan of the Space Wolves. To me they're just ok. I'll always be a true Son of the Lion! The Dark Angels are a Chapter of warrior monks in the best tradition of the Knights Templar. They are known for exceptional strategic planning as a hallmark of their Primarch, the Lion. They share the Wolves fierceness in other ways though, such as their legendary Stubbornness which makes them incapable of fleeing a fight. Personally I always looked on their rivalry as one of a deeper understanding and respect more than as one of just bitterness over a long dead misunderstanding.