Flashes were developed so a standing martingale could be used without having to caverson and drop noseband. Drop tend to fit better IMO, and are more effective (without having to be done up tightly).
i aksed the same question in this forum yesterday and someone provided a link to a good explaination of the differences - will only be a few posts down.
I think i would like to try a drop on twig - i just need to find someone to borrow one off now