The Senate seems to be a much closer race. My guess:
If one of those close races go Republican instead of Democrat then we'll have to wait for January to find out who controls the Senate.
Who controls the Senate will have a huge effect on policy going forward.
If Biden wins and Democrats control 52+ senators, there's a decent chance that they end up killing the filibuster and pushing through significant legislation. Even if they don't kill the filibuster they would use reconciliation to do a couple important things.
If the Senate stays in Republican hands, the Supreme Court and federal courts will stay as they are now and essentially nothing will get passed. Biden will only have executive orders and federal agencies at his disposal.