Darden is a great and very well ranked program. What is it that you feel you have to gain from HSW? Do you think you'll have a better shot at MBB? A better network? Better academics?
Ask yourself those questions honestly and carefully. If you give up a full ride at Darden this year, I would imagine they're not going to admit you again. So, what would your back up plan be? Do you feel confident that you could get a lot of money at another top school? Personally, I wouldn't risk Darden unless you have really solid reason to believe you could do just as well next year.
FWIW, i know a lot of people who got into great schools with big scholarships, who interviewed at Harvard, felt it went really well, and were still rejected. It might be nice to think "oh if that had gone better..." but don't fool yourself into thinking Harvard's a shoe-in, even if you're a very good candidate.