1500 is a classic already, the 1800 is ugly and will never be a classic !
Who is that says an 1800 is less reliable than a 1500? They have everything going for them, fuel injection (proven over a few decades to be a much better fuel system). An aluminum frame which will never rust. Easier to service. Lighter, faster, more efficient, handle better, better brakes. No particular problems that won't happen to any other machine. The only thing better about a 1500 is roominess.