but, prep again is the key. I would power wash both sidings thoroughly. If needed, treat any mildew or algea or any growth.
Consult a "paint dealer" such as Sherwin or Moore and find a good industrial strenght metal primer. Prime both surfaces then you are on your way.
Considering your location, you are getting into a season or time of the year that exterior painting is becoming shaky. Remember, latex paint needs a curing time even though the feel of the paint is dry. 24 hours after application is the curing time. If that nights fall below 50 degrees or so, the curing is compromised.
I would wait until the spring of the year to begin this process.
In answer to other questions, I would definately spray on the primer and finish coat. This would make for a nicer finish and easier due to the nature of the siding.
I would also go ALL latex on your painting project.