As in all warcraft, there are different purposes for different
vehicles. The Starfury is still considered a front line craft by
Earthforce (and a lot of other people, evidently, as the Black Omega
Squadron uses it as their weapon of choice). There are upgraded
versions of the 'fury still being built and used. In fact, the retrofit
package is one of the things that the B5 armorers would dearly love to
get their hands on now that the station is embargoed.
Since the Thunderbolt is designed to handle atmospheric flight as well
as space flight, some compromises were made making it a different beast
to fly in space. It is equipped with the latest in offensive and
defensive systems, and has more to keep under control than the 'fury,
hence the two person approach. Control freak pilots don't care for the
'bolt, as it takes two to operate. It does have heavier armor, as well
as the heat shielding that comes with atmospheric incursion.
There are other ships in the EA arsenal that are of slightly different
characteristic, like the Badger assault fighter and the Olympus class
Corvettes that give a wider range of capabilities of a pretty well
I suspect that the Earthforce arsenal is made up of the pet projects of
dreaming generals, greedy contractors, and plain old visionaries much
like ours is now. This means that there is a wide range of hardware yet
to be seen.