I believe IAAS is the better service in the following aspects.
Promotions: In Railways, you compete with something like nine different services after a certain level while IAAS provides timely promotions.
Diversity: You are almost entirely confined to Railways in IRTS while in IAAS you can work in almost any government department including the Railways. There is a separate Deputy CAG responsible for auditing Railways in IAAS.
Overseas opportunities: You get to visit many countries as part of Audit, while in Railways you don’t get to do that.
Contacts: IAAS helps you network better in the government while IRTS doesn’t have that much exposure.
Nature of Job: IRTS is a field job and is very hectic while IAAS is a very chill service with a balanced work load.
Unless you have a very strong reason to not do so, keep IAAS above IRTS in your service preference.