If you're going to start watching Arrow, watch the 2nd season first. It's by far the best, there's a season 1 recap episode floating around the net somewhere. Starting with Season 3, it does cross over with The Flash, so if you're not watching both shows you do end up feeling like you're missing out of something.
Flash is more of a fun show, The Flash's rouge's gallery is full of utterly ridiculous villains. Instead of trying to reinterpret them as something serious, it just rolls with it as is (Telepathic Gorilla? check Giant manshark? check)