If it's to be your early work - unexperienced - and you get to choose, I strongly suggest a short story. The pain of failure will be lesser.

For an unexperienced writer failures are inevitable. Nobody is born with skill of writing great fiction. Everyone starts with writing crap.

If you think the idea is awesome, the best I can suggest is shelve it for now and write it in some five years, when you've learned the rules of the trade and are capable of making it shine. Don't waste it on what has more than 50% chance of turning up utter crap.

Write a lot, publish online, accept criticism, improve. Experiment a lot and fail a lot. And never worry what becomes of your first story. It may be something to remember fondly or something to be ashamed of, but I don't think it will be a great success...