Benefit: virtual lets you run both at the same time, while with dual boot you boot back and forth.

Cost: virtual always costs more (sometimes a lot more, sometimes a little more). Whether that is a performance difference that matters to you depends on your config and apps. Also a very few "not quite legal" apps -- like the ones that won't port from XP to Vista -- also fail under virtual. So your apps may or may not work right in virtual.

Bottom line: dual boot is safer (you effectively have 2 separate systems). Virtual is more convenient but does not work 100% and degrades performance between 1% and 90% (usually pretty good performance but worst case can be gruesome).