My ST205 has rolled 200 000km, and i have a spare differential with half of that milage. Is it worth to hold on to the spare one, or is it unlikely that my current will be destroyed any time soon? I have a buyer for the spare.

Couldn't hurt to hold on to it.
How accessible are differentials where you are? If they're easy to get your hands on, sell it, if not, hold on to it

They are actually quite hard to come by here, so its worth quite a lot. 4-500$. But then again it would be equally expensive for me if i break mine later.

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So then you should just keep it in case something happens to yours...
No point in selling it if you might need down the road. It's not an overly large piece so it doesn't take up much space.

I have decided to switch the "new" differential over to my gt4 and sell the old one.

Is it as simple as bolting it right up, or do i have to fill oil somewhere? I noticed that it came some oil from the joint between propeller shaft and differential.

Its a differential so yes it does require gear oil. What weight, quantity and where to find the fill hole I do not know.

I imagine the gear oil will have leaked out without the half-shafts installed, so make sure to drain and refill before driving anywhere. If its LSD these sometimes require LSD additives in the gear oil to keep them from making noises.

Hopefully someone more familiar with GT4's comes along.......

Id be skeptical on doing that as well if you don't know the history of the diff.
How long has it been sitting? How many miles/km's on it? How was it driven before, it could be beat to ****.

Sounds like you're trying to make a quick buck, and that usually ends up biting you in the ass in scenarios like this. I personally would keep both, as it won't be worth the 500 bucks if one blows and you're out both diffs then. Just my opinion.

Sell it and use the money for other things. Never had an st205 diff fail. Had a few of them with well over 250ks that have been neglected. Difficult to break. If you did helis or somthn regularly yes i would keep it as a spare..

St205 diffs are pretty reliable in every day use, mine lasted well over 200,000 miles. But conversely they don't always last long if you launch one from a standing start, some last lots, some don't even survive the first launch. If I were you I would keep hold of the spare just in case.