The higher the number..UD3P, UD4P , UD6P ect. the more features and the higher the price. Basically it comes down to the number of PCI Express slots and the number of graphics cards you plan on running.
Asus is also a great brand. I personally like Gigabyte but it really comes down to the brand you are familiar with. Either Asus or Gigabyte are the 2 best in my opinion.
Edit: And Maziar beat me to the answer