OK, I have read your responses (thanks ) ) and I also have done some homework reading and the message I get is that:
You lose some light with both.
Both give a "magnified" image, though there may be some image degradation with a teleconverter -depending on its quality.
Extension tubes result in a reduction of "working distance", whereas a teleconverter retains the lens' original "working distance". (This could be good with live subjects and from the point of perspective and lighting.)
Is there anything else?
With regard to the "light loss": is this "automatically" compensated for by TTL metering for both tube and converter?