You could even argue the most idiomatic C# is things.Sum(x => x ?? 0) and there the real test would be if the compiler/jit would match the speed of the for() loop exactly, i.e. not allocating any enumerators on the heap, not boxing any values and so on.
unfortunately LINQ-to-objects doesn't have the greatest reputation for generating great code/perf, due to a number of factors (largely due to historical impl/interface decisions, from what I understand)
