Winning a bike is really fun!

Do you get to choose the size? It sounds like it from your post.
Some things to consider when thinking about keeping it or not:

1. If I don't get to choose the size does it fit me (or the hubby?)
2. Do I want/need more than one bike? Most folks here would say you can never have too many bikes and I think having a rain bike is a nice thing.
3. If I don't want multiple bikes, will I be able to sell the old one?
4. Can I afford to / do I want to spend the money to build it up? I recently won a frame/fork/headset too, but it still cost me about $900 in parts and labor to finish it off and I already had wheels and shifters to put on it. I'm sure you can do it cheaper than this, but it still takes some money. If you can transfer the components from your old bike to the new one that of course will save you a lot of money.
5. If I sell it instead of keeping it can I get enough for something else that I really want? (a new trainer, a really nice jacket, new Sidis........)

Of course when someone comments on your new bike it is really cool/fun to be able to say Yeah, this is the one that I won......