While both schools are highly competitive and well-respected — the Princeton Review ranks the the Ross School of Business as a top business school in the Midwest and UVA Darden as a top business school in the South East. Their respective full-time MBA programs ranks as #8 and #10 on Bloomberg Businessweek's rankings. The average starting salary of their graduates is basically the same. You can play the numbers games all you want by comparing the figures on their Noodle profiles, research MBA programs in different ways, but when it comes down to it, your career goals should shape the view through which you evaluate schools.
The professors differ, the course structure differs, but what is probably the biggest difference (as reflected in the Princeton Review ranking) is the location. Although both schools have strong national reputations, Michigan is closer to Chicago and Detroit, UVA closer to Washington and Richmond — graduates tend to flock to those respective cities, and that's where you'll find a particularly strong alumni and recruiting presence. Work backwards from your long-term goals and go from there when comparing two schools of equal value.