Both these suburbs have affordable entry points with median prices around the $300K mark. Rental yields are 5.4 - 5.5% which is pretty decent historically. Both have fallen in price since the 2004 peak but Marayong has had a small increase in the last 12 months and also has a higher long term GC of nearly 8% compared with Doonside of 7%.
Rental vacancy rate is 1.3% in Doonside too - very tight. Marayong at 1.7% also tight. Balanced is normally considered to be 3%.
On the numbers alone, there is not much in it - Marayong slightly better.