Partly true, tank killing does not necessary have to be a tank
vs tank thingy. This is the very reason why SAF has long have ATGMs
even before neighbourning countries have MBTs (beside Thailand)
which is never considered a threat to Singapore anyway.
However, tanks vs tanks WILL bound to happen, and that particular
moment, I'm sure all the tankees involved would rather be in the
better armoured tanks than the lightler armoured one. The
penetrating power of the tank rounds is also very important. Don't
want to be the situation like the US tankees during WWII finding
their sherman's gun couldn't penetrate the German Panzars and
Tigers.
I still reckon that SAF should still replace the SM1 (although not
necessary on a 1:1 basis) with a decent light tank with a good low
recoil 120mm gun (like the Thunderbolt). As for the Centurion
replacement, it could be easily fulfilled by Sabra 2 or M60-2000,
(or Lelerc if we have got the money 
)