dazdaz,
Windows Defender and Avast are likely to have some overlap in detection; that is, their databases will each share certain items.
I've heard of Defender (very occasionally) catching stuff that Avast missed. It's not considered a top notch antispyware by most, but has its moments, apparently. I wouldn't know; I haven't run it since it's much earlier incarnation as Giant AS, prior to purchase by Ms).
But if you choose to keep it, and have it run at start up, there is no reason not to keep it updated.
If you keep it running, but don't bother updating it, it's just an anchor. Useless except for contributing a little to drag.