It's hard for me to remember all the nuances and differences between the rivalmance and the friendmance when it comes to Fenspressions. Much of the expressions and dialogue is the same actually. Still get the puppy eyes and smiles and drunk!Fenris.
The main differences are post Hadrianna and the act 3 post Danarius scene. I love the friendmance above all else, but in the rivalmance there are scenes and faces I just adore.
The one in Act 2 where he talks about "This HATE..." The animation is different on rival (maybe its the dialogue choice), but where he looks down and gestures with his hands. I love the emotion in that.
See:

In act 2 as well post Hadrianna the "we don't always see eye to eye..."

Also, where he asks you to find Varania -- the whole "I need someone who can fight to back me up.."

And, bestest of all in Act 3 -- "Yet I respect you.. clearly you are doing something right. Perhaps you see what I cannot.." <-- one of my favorite pieces of dialogue ever.

So yeah the rivalmance grew on me. I didn't want to do it at first, but after I did it I really found it to be very satisfying.
My Fenris album on photobucket has gotten over 2000 views *shifty eyes* apparently people like my screenies. Muahaha.