Both of them were just horrible OSs that Microsoft pulled out of their ass, but I'd rather stick with Vista. Vista doesn't have an oversized taskbar, Vista isn't as laggy as Windows 7, Vista doesn't have as much default programs, Vista isn't as complicated, and Vista is more user-friendly.
Use XP or 2000 instead. They both pwn Windows Vista and 7 by a mile. Or, use Linux (specifically Ubuntu), it pwns all of them.