A Dysarthria is always evident in both reflexive and voluntary motor responses.
A Dysarthria is a condition due to paralysis (either flaccid or spastic) of the muscles of speech, Hence, for any motor movements, whether reflexive or voluntary, the disability will always be evident.
Speech, which is voluntary will be effected, but so will relatively involuntary movements like eating.
An Apraxia is a condition arising from an inability to retrieve the correct motor patterns for a movement.
It can involve various parts of the body, such as the speech mechanism, the arms and/or the legs.
The movements can typically be made correctly if they are reflexive, but become mired in errors if they are voluntary. For example, if I have an arm apraxia and someone placed a realistic plastic spider on my shoulder, I would probably brush it off with my hand in an instance.