The second form always creates an empty map.
The first form is a special case of a map literal. Map literals allow to create non empty maps:
m := map[bool]string{false: "FALSE", true: "TRUE"}
Now your (generalized) example:
m := map[T]U{}
is a map literal with no initial values (key/value pairs). It’s completely equivalent to:
m := make(map[T]U)
Additionally, make is the only way to specify an initial capacity to your map which is larger than the number of elements initially assigned. Example:
m := make(map[T]U, 50)
will create a map with enough space allocated to hold 50 items. This can be useful to reduce future allocations, if you know the map will grow.