Check your local building codes for things like setbacks and easements - you want to make sure your design fits within the code before you apply for a permit, to save time. This also will help you assess details from contractors.

I built a deck myself, using a composite for the decking and pressure treated wood for the steps and railings, so I can't speak to the bidding process for a deck, but in other projects the thing I look for is details - materials used, time schedule, etc.

If you live in an area that gets snow have a open gap at the bottom of railing so you can push snow off easily.

My sister just had a deck rebuilt using composite. The railings were wood, are now black wrought iron style metal. The metal railings look so much better, practically disappear.

Composit gets quite hot to the touch in the sun . . . something to consider.

Our composite decking is a light color, so it doesn't get hot in the sun.