If you're going to offer answers/advice, PLEASE read beyond the second sentence and subject line so you understand the actual question.
I'm wondering if there's an actual difference between the three classes (tactical, science, engineering) beyond kits, ground skills, and the ability to train certain types of officers. Since all the career paths share the same space skills, does that mean a science-based captain can do just as much damage as a tactical-based captain in space (assuming the same space skill selection and identical ships/equipment)? Or do tactical captains have innate damage modifiers which aren't available to the other career types, which in turn makes them do above normal damage with their starships?
I've seen comments which make me assume this is the case. For example, people say an engineer-captain on an escort vessel makes for a reasonably deadly, yet tough escort. The same comments suggest a tactical-captain on an escort is a glass cannon - all firepower and no survivability. Is it the career selection that makes this so? Or are people just assuming tactical captains will focus on damage-oriented space skills while engineering captains will focus on defense-oriented space skills?
At level 5 you get a carreer-specific ground ability. At level 8 you get a career-specific space ability. 10 levels later you get another ability of the same type.
In a nutshell that means extra space abilities which are totally career dependent. See http://sto-intel.org/wiki/Player_Abilities for the different abilities. They are not end-all-be-all, but they do fit specific flavor even in space.
As the others have said, you get a few special abilities for your career. This can help you focus or diversify more, depending on what you want.
Like if you want the absolute maximum damage capability, you would want a Tactical captain in an escort ship. The Tactical captain abilities would give you a little more oomph to your escort damage than the other classes. However, if you wanted to trade out a little bit of that firepower for a little more survivability, you might put an Engineer in an escort. Same for a Science captain, if you wanted to trade a bit of that damage for a bit of utility.
Overall, class choice doesn't seem to make as big a difference in space combat as BOffs and other things, but it is one more thing to consider in how you want your build to work.