I agree that Navy is the easier way to start off fresh.
If you play Psy-Ops, you better become proficient with your newest best friend ever, the Wrench, and your second best friend, lots of prayer and respawning.
(Not to say that Psy-Ops is weak, its just that the Wrench becomes your 'go to' when out of psi power unless you put some skill into a different weapon. Like a laser gun or alien weapons...)
Marine is all "Skulls for the Skull Throne! Blood for the Blood God!" and okay for people that just want to watch the Van Braun burn.
(Marines end up having the most strength for carrying all those bullets, rockets, heavy armor, and such most of the time. Chunky salsa time anyone?)
The great thing about navy, is that when it comes to 4person multiplayer, Navy has two main roles it can fill (Repair Monkey or Hacker) opposed to the one main role for marines (Heavy Weaps man) and one role for Psi Ops (Mind Magic)
In Solo Single Player, Any Branch can technically dabble in the others, but Navy has the easier time dabbling due to being mid-way between rawr-blam-blam and mind-pew-pew.