Ask specific questions about preparing for a science fair, such as: judging, how to set up your display board, preparing a presentation, preparing for questions...
So, you can construct both types of bridges and see how much weight they hold.
Note also there are several types of arch bridges, and you can experiment which type holds the most weight.
I can relate to this project because I fufilled a similar task about a year ago. I would recommend that you be like the ancient romans and build your bridge with a series of arches. Arches are designed to even out the total weight placed upon it, therefore spliting the weight load proportionately. It's worth noting that you should take precaution when contructing arches because if they are built off at a significant angle, the infrastructure's performance may suffer. However you decide to construct your bridge, make sure you are consistant in your decisions *That is the key.