As was mentioned above, successfully writing the ACSO exam may make your application more attractive. If you were DEO and only wanted pilot then you'd obviously not want to or not care about writing the ACSO exam. The difference if you go to RMC is that you will be enrolled without an MOSID; you will be assigned an MOSID grouping (Pilot, ACSO, and AEC are in one group, Air Operations). You are assigned an MOSID after 1st year. You will be ranked against your peers in your grouping and offered MOSIDs based on where you ranked and what MOSID you wanted and/or met the eligibility criteria for. I can't say specifically how the selection board for RMC enrollment works, but I would imagine that an Air Operations applicant that met the eligibility criteria for all three MOSIDs would, all things being equal, be a more attractive candidate. You can decline the MOSID offered and release from the forces without penalty after your first year.
That said, there isn't much to prepare for either the CAPSS or the ACSO exam. The ACSO exam is basically an aptitude exam with an emphasis on math and spatial awareness. The math required is high school level as you'd expect so some review if you need it may help. CAPSS attempts to predict your probability of passing PFT and BFT. The only studying I'm aware of that is noted to have any effect on your ability to pass the CAPSS is actual flying training... one of the reasons that the only way to do CAPSS a second time after a failure is to get a pilot's license or upgrade to a higher level license (ie private to commercial).