As of today means the cut-off time is today. It could mean both depending on the context. For example, you could say "As of today, I still have not received an invitation to John's wedding. It could also mean "today onwards" as in "As of today, I resolve to do my job concientiously.
"Until today" means something is going or not going on until today which the reverse happened. Example, "The kids kept asking me loose change until today (which they did not).... or...."He always kept to himself until today (which maybe he opened up).