I bought the extra space one, but sadly haven't actually played any space content in the last week or two to check it. Been all anniversary events, voth battlezone, or colony invasion. Just grinding Qmendations, DIL, and Marks to feed the projects atm...
Seems like a no-brainer to buy if you've got the fleet credits to spare and don't plan to blow them on a console/ship/weapon upgrade right away. I passed for the moment on the ground slot, just couldn't talk myself into needing any of the extra powers the slot would bring. Space, though, is an easy way to increase DPS or healing
I unlocked both space and ground. They're both very useful for my tac officer but sadly I can't find any suitable ground doff for my sci officer who's a ground healer. So yeah that extra ground slot isn't always useful.
Where do you buy additional slots?
Our fleet hasn't unlock the fleet vendor so you'll have to get an invitation from another fleet who has already unlocked the tier 3 Spire and buy it from their vendor.
Some fleets charge an arm and a foot for the invitation, but the NoP Public Service channel has good people who offer free spire invite if you ask politely.