You have to know both.
If you're trying to cover some of the classic thrash metal catalogue then you have to listen very intently to work out what those guys are actually doing. The difference between all downpicks, alternate, all upstrokes (yes this happens - think about main riff for Cowboys From Hell) and any combination in between are subtle but make a difference. Generally downpicks sound a bit thicker usually because of the lower tempo. Alternate pick riffs are for all those fast bits.