What time zone does Date.toString() display?

It displays using TimeZone.getDefault() which, in turn, will default to the time zone of the operating system it is running on (i.e. the host). So in practice, they will be the same thing

Note that a Java date is not really a date! It is an instant in time, represented by a millisecond offset since the beginning of the epoch. It still contains methods which reference year, month etc but these are all deprecated. You should on no account be using a Date object as if it were a date.

Use a Calendar (although this is arguably even more broken than Date) or a decent library like JODA.

Does it display the default time zone for the host, or TimeZone.getDefault()?

The latter (which is the same as the former unless you set it explicitly somewhere). However, this is information gleamed from the source code, so it should be considered an implementation detail. If you want specific repeatable behavior, you should implement it yourself.