using the same time on both will force immediate updates.
Setting them a few hours apart will spread the updates over that time.
The first is like pusing to ALL clients at one time.
The second spreads the load and updates them at random during the time period specified.
HOWEVER, I've used BOTH and STILL find that some simply don't update/upgrade until I push using the tool.
There's always a few that until you reboot them, or use a push install, they just won't use that "assigned" upgrade method.
It's actually a small, but still frustrating amount.
I don't know WHY that using the "do it ALL NOW" approach works better than spreading it say, from 6pm to 5am over several hours, but it does.