Neither of them. Both are great forms. I love flash fiction for its speed, economy and how every word must count. It's a bit like taking a really short refreshing drink. It's to the point. Short stories are more like taking a longer refreshing drink. You get more, you enjoy more. While there is still not the room to do in depth characterisation, you can get more across in 2000 words, enough to add depth to the story as a whole. But for a snapshot of a moment, you can't beat flash fiction.
Easiest to write?
Standard short stories as you have more room. I find it easier to edit down (which I inevitably have to do). Flash fiction - I find I either go too short (for me flash fiction is 250 - 750 words. All credit to those who can go below 250 but for me that's like limbo dancing with words. There are just some places I know I can't go and this is one of them!). Alternatively I go too long and I find it harder to edit a flash piece, knowing every word has to work twice as hard at least in a piece of this kind of length to justify its place in the story. There is no "slack" (and I know there isn't in a 2000 words piece either but you can allow an odd sentence here and there which backs up your characters, can give a little additional information which rounds off the piece. No room for that in flash).