Answer
They would exhibit less diffraction.
Work Step by Step
Protons moving at the same speed as electrons have more momentum, and shorter wavelength.
Momentum is p = mv, so the larger-mass proton has more momentum. The de Broglie wavelength is h/p, so the proton, with larger momentum p, has the shorter wavelength.
Diffraction is more apparent for long wavelengths (as stated on page 548), so protons would diffract less.
