Trademark cost, state by state.

Reach, mainly. A state trademark is registered on one state's own register and operates inside that state. A federal trademark is registered with the USPTO and operates nationally. They are separate systems with separate records, so being registered in one place tells you nothing about your position in the other. Each state page compares the two for that state directly.

It depends on where the business actually operates and where it intends to. A business selling entirely within one state may find the state register answers the question in front of it. A business selling across state lines, or online to the whole country, is operating in territory a single state register does not reach. That is the comparison every state page here is built around.

No. Forming an entity puts a name onto a state's business register so that two companies are not registered under the same name in that state. A trademark is a different question about the mark you trade under. Plenty of businesses hold a registered entity name and no trademark at all, and only discover the gap when somebody else starts using the name.

A DBA registers a trading name on a public record. It is not a trademark and was never meant to work as one. If the goal is to comply with a naming rule when you trade under something other than your legal name, the DBA is the right filing. If the goal is to be able to rely on the name against other businesses, you are asking a trademark question.

Yes, and some businesses do, usually where a strong local presence sits alongside plans that reach a good deal further. They are separate applications, made to separate offices, sitting on separate records, so holding both means maintaining both and searching both before either is filed. The starting question is still the same one, though: which register actually covers the ground the business stands on today.

Start with where your customers are. If they are all inside one state, read that state's section on when a state registration alone is enough. If they are spread across the country, or the plan is to be, the federal route is the one the comparison usually lands on. Every state page here carries both sections so the decision is made against real detail.