Long bed for sure. The longer wheelbase will ride better. Many models have a larger fuel tank with a long bed. And, you will be able to put more stuff in the bed.
When it comes time to sell a dually lots of contractors might be potential buyers of a high mileage long bed dually to use as a work truck. Not so much with a short bed.
Further, if your buying new GM doesn't offer a dually short bed and I don't think Ford does either. Even in the past when they have they were quite rare.
However, I would look hard at medium duty trucks. You will have a much more capable truck all the way around for probably less money than a new dually.