What do you mean by "faster"? If it means straight line speed, Venom GT is clearly the winner (they have a record at 270+ mph). Agera R does not have any official speed yet. 273mph is just a theoretical top speed.
However, if it means "lap time", it is yet undecided. In my opinion, Agera R has better handling and brakes compared to Venom GT. Therefore, in lap time, it probably would be Agera R.