It's not easy to get into the RSAF but not impossible. Think carefully, your first bond with the RSAF will be 12 years. RSAF pilots retire at 45 no matter how fit you are and depending on what rank you are when you retire, they might prepare a ground position for you, not necessarily with the force. SIA pilots get to fly to 60 (commercial), for freighter pilots (SIA cargo), they fly till 62.
There are chances for you to come over to SIA during your service with the RSAF once your bond is over. The scheme is slightly different from SIA direct entry cadets. SIA throw in considerations knowing that you might already be married and have commitments by the time you finish your first bond with the RSAF (prob around the age of 30-35) so your pay is not as pathetic as the rest of the cadets.
Nonetheless, both are good careers that you should seriously consider.