Winddragon - to add to the other replies;
KFC is basically the same spot as the Train but just a different launch area and slightly further north (same current direction and wind effects)
Wavebreak island divides the train and shearwater and the ocean entrance is level with wavebreak island.
The following is a generalisation, and you do get freak days where they are still sailable outside this;
The Train / KFC - A "true SE" or "NNW through to NE" wind is the ideal direction and allows clean wind to blow right across all the water. Other wind directions result in very patchy / gusty wind, but occasionally you score a freaky day where it still works (rare). It is less tide affected than shearwater but you can still feel the current pushing or pulling you (maybe 1 - 2 knots). When the wind is outside of the ideal wind direction for the Train, one can think there is no wind when arriving at the Train, when really there is a solid 20 knots at the shearwater. Overall, the Train has sailably "flatter" if not "glassy smooth" flat water most of the time. Very high tide is choppy and not flat.
The Shearwater - "S through to SE", and "N through to NE" is ideal, but again you do get the freaky day outside that range that works (rare). The current is noticeably stronger than the train at about 3 -5 knots. Therefore if the wind is marginal, a wind-against-current combo (ideal) increases your wind, or, a wind-with-current combo subtracts from your wind. The water is not as flat as the Train, but does get flat-ish in the middle. (ideal for those learning to gybe as you can stand up in the middle)
Shearwater is all of the GoldCoast's windiest place (with the exception of North Kirra in late winter) overall in stregth and regularity, particularly for wind coming from the southerly directions, but the wrong wind/current combo and it's bigger chop can make it annoyingly difficult at times. It has many large cruisers travelling back and forth on the weekend (week days are usually fine) which makes it extra choppy. Once the wind hits a genuine 25knots+ the shearwater cleans up and has great ramps, the boats disappear, and even in a wind-with-current combo it is nice to sail as you will have plenty of wind to help stay upwind.
If you haven't already realised, a BOM average observation of 20knots = a genuine 15knots (with hand held wind meter). The BOM over exaggerates almost all of it's forecasts by 15-20%, if not more.
Hope it helps