We tried VW senders and resisters, even simultaneously on the same van.
We use resisters  because of the huge range of variance in the calibration
of the dash gauges.
  For a Subie, we usually use a 27 ohm resister.  80% of the time it puts
the needle right on the top half of the LED.  This spot  ensures that the
LED will flash before the engine grenades itself.  We want the radiator
fan to come on before the LED flashes.
   30 ohms is way too much, and 24 is usually too little.  27-28 is about
right.
   The gauges aren't really very accurate.  What you are really using it
for is the idiot light and cooling system "behaviour".  The engine should
warm up, and get up to the temp that the thermostat is set for.  In normal
driving, it should be dead stable at the same point every time.  IF you
are sitting in traffic on a hot day, the engine temp will rise above the
thermostat temp, until it reaches the temp where the radiator switch turns
the fan on.  The temp should drop slightly, the fan should turn itself
off, and the whole cycle will start again as you sit there.
    External VDO gauges are nice, but not really necessary, even though I
have them on my vans.  They have their own set of weird characteristics.
The temp and oil pressure both vary if you have their little internal
lights on at night.  Temp goes up almost 10 degrees.
    VW temp senders are cheap, we don't use them just because that is our
opinion.  Many others use the VW sender.

    John