Sol contains the only three non-procedurally-generated planets in the game: Earth, the Moon and Mars. Earth is always the focus of visitors, of course, but Mars looks pretty cool too, as it's been terraformed and has rather unique geography, with the volcanoes jutting up clean out of the atmosphere.
There's also a couple of "easter eggs" in Sol system for the persistent egg-hunters to find: the two Voyager space probes, several million Ls away. You need to follow the guides to find them.
Achenar is less interesting, plus has the disadvantage of being a giant star; the interesting inhabitedd worlds, including Capitol itself, are thousands of Ls away from the jump-in point.
In terms of actual gameplay, both Sol and Achenar offer this: five native factions all of the same faction type (five Democracies in Sol, five Patronages in Achenar) so there's never any war there. The NPC populations of both systems are huge, so even a large group of players would find it very difficult or impossible to try to change the controlling factions. Becoming Allied with all five factions takes some work, but doing so can give quite lucrative mission opportunities. Being Service economies and often with one faction or another in Boom, bringing cheaply obtained Medical Diagnostic Equipment from high-tech systems to either of these places can be the most lucrative trade activity in the game. Unfortunately, it's a "dead-header" trade route as Service stations export nothing but biowaste. Which is again a wasted opportunity; I'd like to see the capital planets export unique trade goods, like "ancient artefacts" from Earth and "artwork" from Achenar; these goods are already in-game, yo just can't buy them anywhere. Come on, give a trader some reason to go to these places.
It should also be pointed out that finding missions asking you to go into permit-locked systems still is not a thing, even if you have a permit. Which seems silly to me.