Are systems in the real world either isothermal or adiabatic?
Real processes are rarely completely adiabatic or isothermal - but often approximate to one or the other depending on the time scales and space scales involved.
Extremely rapid events when there is insufficient time for much heat transfer to occur can approximate to adiabatic behaviour - as can the behaviour of very large volumes of gas (eg. moving up or down through the atmosphere) where conductivity is low over the distances involved.
Conversely, for very slow processes or those happening on a small scale, heat conduction is likely to be sufficient to give an approximation to isothermal behaviour.
links:
The adiabatic atmosphere