Both have their ups and downs.
Pivot: Easier to use
Easier to animate (stick figures)
Pwns hard
Flash: More dynamic animations
Is better if you know what you're doing
You can make games
Really, it's all a matter of preference. If you're pr0 you probably want to use flash. If you're an 8 year old who enjoys stick people beating the shit out of each other (the most common cliche with pivot) then you probably want to use Pivot.