I always liked to work from the top down. That would mean roof first.

Soffit/fascia work can be done by either. That work is common to both jobs. If you use a contractor that can do both, that might make a better looking finished job, since there could easily be fascia work needed when the roof is replaces. Doing the soffit work is usually a part of the fascia repair.

Another vote for top down.
Together would be ideal, but time/finances may preclude that. In that case I'd do the roof this year and the siding later on.

I would talk with a contractor about which is in the worst shape. Can one survive through the winter, assuming you're in a colder climate? Also, are there any tax credits for either one--not sure what is left besides windows on the whole energy tax credits that expire at the end of the year.