There isn't a huge difference in economy quality

Is $230 saving worth the extended journey time

The MCT in Melbourne is 60 minutes, so it is possible to get connection of 70 minutes. Leaving at 16:15 from SYD to get into SIN at 23:50 for a total of 10 hrs 35 mins. If extending to a 2 hour connection in Melbourne , that would bring it to 11 hrs 25 minutes

Non stop on Qantas would be 8hrs 25 minutes.

If just paying for myself, I would take the nonstop

I would avoid the 2 hour connection and get it down to 70 mins if choosing to go via MEL; enough time to connect but not so long as to be a tedious wait