Convex buttons are more popular for Japanese machines. They're prefered by some people for fighting games.
Though I must admit I grew up using concave buttons and prefer them for their familiarity, I don't think it would take much time to get used to convex buttons.
If I had to choose between pre-wired convex buttons and basic translucent do-it-yourself concave buttons, I wouldn't have to think too long before ordering the convex.