I usually default to TUN as having it in it's own subnet is better for a variety of reasons, among which firewalling, and OpenVPN basicly works out of the box like this.
TAP has several advantages though, among which is that connected clients behave exactly like physical ones. This means that for example any ip restricted resources that are normally only available in the physical network also work.
Another use case I see in home situations is that the OpenVPN server has no internet facing side and thusly routes through some sort of provider supported modem/router.More often then not these don't support things like static routing which is a must if you let OpenVPN use it's own subnet.