A deck is an open, elevated frame. A porch is the same frame built to carry a roof and, usually, a screen. The real choice is weather protection and cost, not the structure underneath: both go through the same permit process and the same ledger and footing details.
Structurally, a porch and a deck start from the same place: a frame attached to the house or freestanding, on footings sized to the load. A porch adds a roof, which means real added weight and wind load the framing has to carry. Screen a porch in, and the frame also has to hold the door and screen panels without sagging over time.
That is the whole structural difference. Neither one is "better built" than the other. What changes is what the space does for you.
An open deck costs less because there is no roof structure to frame or screen system to build. It is the faster build of the two, and it puts you in full sun and open air, which is what a lot of homeowners actually want for grilling, seating, or a hot tub.
A porch, especially screened, gets used more of the season: rain and bugs do not end the evening the way they do on an open deck. That protection costs more, both in the roof structure and, if you screen it, the door and panel system. It is a real tradeoff, not a straightforward upgrade.
Both go through the same zoning and building permit process, and both start with a check of the ledger and flashing where the structure attaches to the house. Framing and span information given before permit is advisory: final member sizes are confirmed on the stamped plan with the local building authority at permit.
Structural framing discipline (footings, ledger and flashing, span-rated framing) is the same discipline we use across new deck builds and porches, drawn from our own code table and the current Residential Code of Ohio provisions. Sources: Licking County Building Code, Ohio Administrative Code, Chapter 4101:8-5.