Re: military route to SIA vs going to SIA straight
Your question: is it better to wait and join the airlines direct?
In one word: Yes.
Every year, the number of RSAF pilots jostling to leave and join the airline far exceeds the official number (15) who are allowed to leave. This will also only take place when you are in your late 30s. In addition, you will also not be in control of many issues that will / might pop up to frustrate your plans. Sure, RSAF pay is good, the possibility of flying an F16 is exciting etc etc.
Contrast this against being selected for SIA as a cadet. You only have to wait until 26. This may seem like an eternity, but it will pass in the blink of an eye. You go to NS, finish university, get a few years of working experience (SIA or otherwise) and then you will be eligible to apply. The odds are much more in your favour - the factors that are in your direct control, such as your own success in school and at work, and then against other applicants as a cadet pilot applicant. You can further stack the odds in your favour by applying to be a cadet flight instructor with SIA (there is no age bar) after your degree, but as an instructor the pay is not good and you must really have a passion for instructing.
I would compare the relative odds by saying that joining the RSAF as part of your larger plan to go commercial, would be like trying to build your life savings by buying lottery tickets. It might work, but in all likelihood it will fail.
If you are interested in becoming a commercial pilot ONLY, then do not use the RSAF as a stepping stone. You will be extremely frustrated and disappointed.