"Lead was created" is not a single field like "Created at" (which is the create date). "Lead was created" is some kind of log. It's a combination of factors which you can use as constraints (e.g. source, form name, lead source).
I'm not sure, but I think it is possible to update the "Created at" field with a new date, therefor effecting your process. However you can not update logs. So I think "Lead was created" is more fool proof. It also enables you to filter out list imports. You don't want your SF guy to automatically get 10.000 leads synced at once, when someone does a list import.