difference between optocoupler and buffer
The TLP250 you mentioned requires 5 mA input current to guarantee turning on.
If you need to operate 6 of them together, you might need to source 30 mA from the controlling device. A higher current might be desirable to give faster switching (although the datasheet makes no promises about this).

For some microcontrollers or DSP's 30 mA would be no problem. For others, it might violate the output load specs.