You should also keep in mind what the service has to offer should you not get a flight contract. And if you do get a flight contract, if you'd be okay with something other than helos (hard to select for Marines right now). Finally, look at how long the process can take.
For Marines, you have to go to TBS. I don't know what to timeline looks like now, but I have friends who commissioned NROTC Marine Option and had to wait 4 or more months after commissioning to even start TBS. That takes 6 months, then you head down to Pensacola and wait around to start API. That can take a few more months. You could easily be at a full year after commissioning before you even start flight school. The wait between API and Primary is longer for Marines than Navy typically, one of my friends sat around in Corpus Christi for 5 months before starting Primary. Primary takes another 6 months. The wait for Helo Advanced for Marines is sitting at 6 months right now as well. Training is an additional 6-8 months. You can easily be at 3 or more years post-commissioning before you get your wings and head to the fleet.
As far as wait times go, the Coast Guard seems to be more in line with the Navy which is about 1-2 months between each phase. HOWEVER, there are very few CG Ensigns coming through flight training because most are sent to a boat first. The only Coasties I know who got sent to flight training right after commissioning were USCGA grads, and even then there's only a handful of them who go direct. So realize you may be going off to do other Coast Guard things for 2-3 years before you can go to flight training.