The reason that "Tosca" is more compelling than "La Traviata" has a lot to do with the type of heroine and the basic plots of the two operas.
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Maybe you feel this way because Irina is usually so literal in her interpretations, miming the origional rather than just skating to the music, IMHO. Personally, I just don't care for more literal interpretations, like Carmen getting stabbed in the gut at the end of the LP. Didn't like it when Kat did it, didn't like it when Sasha did it, didn't like it when A-P did it with Madame Butterfly either. I'm atually glad Irina wasn't coughing and dying in La T.
As for the two programs, I agree that it was the choreographer. Arena is famous and the level of sophistication in Tosca showed, even if it was more literal that La T, which was pretty darn bland choreographically. Tosca had tension, which La T didn't IMHO.
IMHO, La T is better music, even if Irina chose some of the weaker, less famous pieces.
Personally, I also like Violetta better. She was honest and kind and had a lot of sympathy for others and gave up her own happiness to supposidly make life better for her guy. Tosca was jealous and an annoying diva, and she totally panicked and blew it in the end, jumping off the roof when her guy wasn't even dead. I find her silly.
Anywho, like I said, I just think that Tosca LP had a better choreographer, regardless of the Opera's story.