Ridge vents won't let bugs in, nor any build up. They are cheap and easy to install, and are generally viewed as "better" than gable vents because they span the whole roof. Usually they are coupled with vent openings in your soffits.
I had a similar situation when I added a garage. I took out one cable vent, added a ridge vent, and drilled vent holes in my soffits. I left the other gable vent as it doesn't hurt. Later I added a power vent on the roof. Not because of moisture, but just to remove the excess heat in the summer. My AC is now much more effective after doing that.
If you switch to steel, the venting will be unique to the steel system. I think steel systems also support a ridge vent design.