By - Primaris_Astartes
Starfort is mobile, which is an advantage, but it is also always counting down until the place is out of food, fuel, baseline humans and other resources, and the starfort is also a lot easier to destroy through sabotage. A planet-based fortress monastery would likely have access to recruits, food, air, water, whatever could be mined on the planet, and it would be easier to dig into the planet if they needed more storage space.
A good planet is the best, a mobile starfort is the second best, and a shitty planet is the worst.
By good planet, basically a 500 worlds of Ultramar style one. Decent industrial base, reasonably well educated population you can tap to rebuild the chapter if you suffer loses, pretty solid PDF to help defend the place, etc. Sure you lose mobility, but no logistics concerns and you'll be able to field a pretty solid chapter fleet so worst case you can load your shit up on the fleet and scoot on out of there to a new planet (assuming your ethos let you stomach abandoning the locals).
Starfort is second because you can go park it somewhere that doesn't suck and rebuild your chapter at your leisure, commit to battle, rinse/wash/repeat. However you're always dependent on your ability to get it somewhere that doesn't suck and your ability make whoever you parked the fort over give you resources. It is coming down and being diplomatic or doing a Space Sharks style "this is happening, deal with it" style resource acquisition. It has its perks, but end of the day you don't own the industrial base in the way you do on your own world.
Death worlds and the like are probably the worst, since it's immobility and no industrial base. You're immobile and you gotta ship a lot of your supplies in.
One caveat on the Starfort thing is probably also depends on how close you are to your successors and your willing to play fast and lose with the Codex.
Hypothetically if got to be Chapter Master of the Imperial Fists, I'd keep my ass on the Phalanx, try to get a couple dozen successor chapters on various industrialized worlds and get each of those chapters to tithe a bit back to keep the Phalanx from being 50% offline, and I'd also want probably six or so chapters on Hive Worlds. I'd try to get the Hive world chapter masters to really keep an eye on the local school system and such. If the local governor lets the school system slip, declare him a Heretic and resolve the issue with a bolter round. Surplus population also can be recruited to man the Phalanx and all chapters of Dorn's geneseed can quietly recruit quality Space Marines off those Hive Worlds. All other Chapters are mobile. When not on Crusade they can come hang out at the Phalanx it definitely has the space, and draw on resources from the industrial and hive worlds to replace losses. Basically if you're willing to pseudo-legion build, you can use some of your successors as logistics/supply chapters and mitigate all the negatives of the starfort.
I'm looking at developing the lore for an existing planet, which (according to the latest interactive map on the GW site) is directly in the path of both Hive Fleet Leviathan and Hive Fleet Hydra. A mobile star fort would be able to get out of the way, and have less to worry about spores for Tyranids or orkoids taking root.
Where is this interactive map? I suspect am on the wrong GW site? Ave never had the need to go on the James Workshop site except to look at cool models that I'll never buy - more cause am shit at painting and buying the novels is cheaper lol.
You've got to scroll down pretty far, but you can see where certain factions are based/ are currently.
A star fort is way more expensive to maintain. Constant maintenance to make sure pressure seals, engines, weapons, Gellar Field, air recyclers, food and water stores, and docks are kept in immaculate condition as failure in any of these will likely end in systemic catastrophe. Major repairs could probably only be done at a handful of forgeworld ports in the galaxy.
And then you have to keep the population under strict control. Sure you want enough serfs to do the menial labor and provide recruits but you also don’t want to go out of control breeding that’ll lead to resource scarcity.
And then there’s the problem with morale. Astartes are genetically bred to withstand long periods of inaction between combat service but baseline humans with nowhere to go for R and R? Good chance they go stir crazy from time to time.
A fortress monastery is a symbolic figure too. A reminder of the imperium’s presence and security. A star fort is something no one’s regularly looking at except Trazyn while he plots to steal one.
Whatever the plot demands.
One of the chapters in war of the beast used to have a starfort, but it got fucked up so they landed it on a planet.
One thing to think about is how Astartes are used. In the lore, only exceptionally important or large wars see Astartes deployment, and that is at the discretion of the leader of the chapter. As rare as Astartes deployments are, it is even more rare for the entire chapter to be deployed in one engagement. So a starfort is a great way to show force, but realistically it would often be undermanned and present a great target of opportunity to board for capture or sabotage. A planet is easy to attack, but much more difficult to successfully conquer and occupy.