Is there a functional difference between the following? Is there any reason to prefer one over the other?
auto p = new C();
and
auto* p = new C();
In the snippet you have given there is no difference. The compiler will deduce that p will be a pointer to C, either way.
In a more general case, there is a difference. For example;
auto *p = func();
auto p = func();
The first form will cause an error message if func() returns something that is not a pointer, but the second will not. This is sometimes useful (e.g. in templated code) where there is a need to enforce a requirement that func() return a pointer rather than (say) an int. [Although, admittedly, there are more clear and powerful ways to enforce such a requirement, such as traits].
As noted by John Ilacqua in comments, there are further benefits of using auto *, including const qualification of the pointer. For example, const auto * p = func() specifies that p points at something that is const or const auto * const p = func() specifies that p points at something that is const AND p cannot be reassigned. Such qualification will also result in diagnostics if const qualifiers on the return type of func() are incompatible.