Depends on the spindle. Vipers adapt to drum spindles, Wilwoods adapt to disc spindles. Both are designed to match the 11.75 rotor, so should fit most 15" wheels. I've checked numerous wheels, to include OEM steelies, and haven't found anything they won't clear yet, but some aluminum wheels from the '60s and '70s were drum or disc specific, so I'm sure there are wheels out there they won't fit.
Viper caliper is larger and more rigid, so it deflects less, but this size can cause interference with some wheels. They are also OEM engineered for long life and minimal maintainence on the street, but rebuilds can take some time or dollars to complete.
Wilwoods are small and lighter, will fit more wheels and have a large slection of pads from multiple distributors. They can be rebuilt quick and easy, but the lack of dust seals means you need to check them more regularly if used on a semi regular driver.
I can't say either one is a better performer than the other based on all teh feedback I've seen.