I'm inclined to think that, historically,
take care so that may have been the original form, and
take care that grew out of it. Is there any evidence to show this?
Still,
take care so that seems to say
be careful for the purpose of ... or
be careful in order to ..., while
take care that seems to say
be careful about ....
In other words, the first seems to focus on the reason for being
careful, and the second seems to focus on the subject matter of the
care.
CJ