FS = Flight Surgeon, DMO = Dive medical officer. Flight surgeons are essentially GMO's for a squadron of Navy or Marine Corps pilots, DMO's are the GMO assigned to the sub community.
Honestly they are both nice gigs, im a flight surgeon so I am kind of biased towards that one.
If you did flight surgery, after intern year you will "stash" in a clinic at your intern-year hospital for a few months before heading to flight school. While stashed you basically just see basic clinic stuff with attending supervision, its super chill. Once at flight school, you dont do anything clinical. You basically get to go to school with pilots for 6-8 months and get to fly, its awesome. Days are from like 8am-3pm Mon-Fri with lots of time off. Flight school is pretty easy, they don't want you to fail and you aren't expected to perform at the level of the actual pilots (except for a few weeks during something call "API" but its not difficult). After that, your 2 year billet consists of you working 2.5 days a week in clinic and the other 2.5 days you spend hanging out with your squadron pilots/flying with them. Its all preventative medicine with the occasional weird thing but there are plenty of people you can always call and ask for help. You get trained to deal with aircraft mishaps which is cool to be a part of. You will likely deploy once during those 2 years (although depends on the billet, you may not deploy at all). During intern year you will make about 75-80k for the year, once you finish intern year your pay goes up to about 100k/year which continues through flight school and your 2 years as a flight surgeon.
And then once you are done you can go off to civilian residency with an awesome life experience and a killer residency app. You can use your GI bill to supplement your civilian residency pay and make about 20k/year in addition to your civilian residency salary.
For people looking for that "military experience" as a doctor, this is the way to do it. Med school gets paid for, make a decent salary for a few years actually on the front lines of the military for a couple years, and then you can go on with residency and the rest of your life as a civilian. Do you lose a few years? Sure, but I guarantee you there are no 65 year old doctors looking back on their life and said "man, i wish i had 3 more years as an attending when I was younger". They are saying things like "man, i wish i had learned how to fly, or do something cooler when i was younger". This is the way to do it.
You don't really have a monetary advantage, but you will break even with your civilian counterparts. For example: lets say you were able to pay for your first year of med school. the other 3 years are about 150k (from military paying for tuition) + 80k from your stipend = 230k value. Intern year is about 75k gained. Flight school + 2 flight surgery years = 300k. Now you can start civilian residency, but you make 20k/year more from GI bill benefits for a standard 3 year residency (lets say 60k is normal salary, so you make 80k/year). Add it all together its about 845k earned over that 10 year period. Now if you were a civilian the whole time, lets say you again were able to pay that first year on your own. You incur 150k debt the next 3 years. you get paid average resident pay (lets say 60k/year) for standard 3 year residency. Now you get your 4 years of attending pay in the same time it takes the military person is at (new attending lets say you make 200k/year), so add 800k. Comes out to 830k. Both are similar, and obviously it will differ for people based on where they go to school and what-not but essentially, the monetary value comes out the same. The whole point is that you won't be ahead of anyone, but you will get a cool life experience and it's not going to set you back monetarily.
And about the 20k bonus, i would double check with your recruiter on that and have them actually show you the fine print (dont just take their word). That was a thing back when I signed on, but it could have changed since then.
