Also, Subs never need biospheres, and so they will be able to use hydrofarms, which are sweet tools in the early game, even if biospheres are more important later. This helps speed your growth by freeing up a farmer per world, at a negligible research cost..
Well that makes me think a lot about it. Maybe you can help me to find my mistakes....Your proposed hydros are junk. Almost noone considers them. 2BC maintenance simply hurts - even for sub. There is only one race where I would take them. Thats Demo Sub art hw.
Biosphere => 80 research, 60 build, 1 maintenance
Hydroponic => 80 research, 60 build, 2 maintenance
Now, let's take a closer look.
Biosphere allows you to have two more colonists per planet. Without any bonuses for farming, this means that 1 of these colonists have to be a farmer (terran = 2 food). The other one can be used as a worker or a scientist. After all this building gives you +3 PP or +3 RP... as you wish.
Hydroponic farms on the other hand, gives you +2 food, so you can change a farmer into a worker / scientist, giving you the same profits.
For that, it seems quite the same result!
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But let's assume you have 3 planets ... 1 terran (12 pop) and 2 barren (4 pop). Normally you can't build farmers on barren enviroment. To save you from starvation, you have build freighters to send food to the barren homeworld. So I also assume, that you built the new tech on any planet.
With Biosphere, you can have a population of 14 + 6 + 6, so you need 13 farmers to keep them well-fed. So you have 13 colonists for your disposal.
With Hydroponic farms, you still are at the original population maximum. Therefore you have 20 colonists. Since you get 6 food due the farms, you only need 7 farmers. So you can use 13 colonists too.
Still sounds quite the same? Let's get to the maintenance cost:
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Each Biosphere cost 1 BC of your income, it results in 3 BC for our example. Hydroponic farms however have a 2 BC per turn maintenance cost, so as a result you have to spend 6 BC per turn. BUT!!! Let's take a look at the freighter cost you have. Each freighter needs 0.5 BC, if used.
So the Biosphere strategy has to spend 6 BC to fed the 12 colonists on the barren colonies. The hydroponic farm player spends only 2 BC per turn, because he only has to fed 4 colonists via freighters. (per barren: 4 pop - 2 due farms = 2 colonists to fed)
So the player with biospheres has to spend 9 BC per turn and the player with hydroponic farms only has to spend 8 BC. To progress with it, ny player with biosheres hast to spend 4 BC per barren planet (3 BC for freighter + 1 BC for biosphere). The player with hydroponic farms only has to spend 3 BC per barren (1 BC for freighter and 2 BC for farms)
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To get to the differences:
With Biospheres, you can handle more population, so more population, results in more taxes. So in the end, you will have more money. If you have a farming edge or get the right tech, biospheres are better too.
But on the other hand, hydroponic farms gives you an instanteous effect. You don't have to wait until you get the pop up to the planetary maximum. If your star system is blockaded, you might not need to worry, because starvation isn't as high as with biospheres. And you have more colonists free on your terran world (1 terran + 1 barren). So it's the best choice, if you have a farming disadvantage.
If you reduce the maintenance cost to 1 BC (like the DifficultChoice mod does), it get's even better for the hydroponic farm. Alternatively, you could increase the maintenance cost for biospheres (my favorite) OR double the cost for freighter use to totally negate any advantage for biospheres. An other side-effect would be, that it get's more expensive to send colonists around the galaxy, so the 1 pop housing strategy would cost you 5 BC per colonist per turn of travel, instead of 2.5 BC.