PlayStation face buttons (cross, circle, square, triangle) are flat, require slightly less force to press in, and have a matte finish (up to the DualShock 4, at least).
Xbox face buttons (A, B, X, Y) are round/convex, require slightly more force to press in, and have a glossy finish.
Which do you prefer and why? I enjoy both but I'd give the edge to PlayStation because I find them much less fatiguing to mash.