Ukrainian or Russian
Ok, so I obviously know that Russian is spoken more, but is Ukrainian (or at least the course) easier than the Russian course? Which course should I do, as I have found Russian rather challenging.
The Ukrainian tree is a lot shorter. I plan on finishing Ukrainian before getting serious about working through the Russian tree.
As far as grammar goes, Russian and Ukrainian are basically equally difficult. I'd guess that Ukrainian has a slight advantage for English speakers, since it seems to have more loan words from Western languages. The Ukrainian team doesn't seem to be as active as the Russian one. Lots of faulty translations/audio and missing audio that haven't been addressed it. So it might be better to keep with Russian until the Ukrainian tree is improved. Whichever you choose to start with, you can always finish the other later. Switching between shouldn't be too hard, the biggest difference is vocabulary. One advantage the Ukrainian course has is that it's already available on the mobile app. Also, it uses a real human voice, so it sounds more natural (the only downside to this is lots of words without audio).
From my experience, Russian and Ukrainian are approximately equally difficult. At least, the things about Russian that are hard for English speakers (like learning which cases to use, remembering all the endings) are conceptually about the same amount difficult in Ukrainian.
I am looking forward to trying some more Slavic languages to compare though!
I think it depends on how serious you are about learning the language – is it a nice to have for you, or are you actually planning on speaking it?
As a Russian speaker, I find Ukrainian fascinating and am learning it right now. However, Russian is not simply more useful in real life; it also has significantly more resources. Just an example: as part of learning Ukrainian, I tried looking for a print English–Ukrainian dictionary, and found either useless tourist-oriented resources, or super-expensive specialty editions a la comprehensive Japanese dictionaries. There are also significantly more resources on the Internet for Russian than for Ukrainian, and most Ukrainians speak Russian while most Russians do not speak Ukrainian.
On the other hand, Ukrainian is also close to Polish, so if you intend to learn Polish in the future, Ukrainian will make it far easier, while Russian will not help much.
For a foreigner Russian would help with Polish as much. The missed similarities aren't the hard things.
In my opinion, for a Russian speaker the best way to learn Ukrainian is to read a lot, I think (if you can't go and live in Lviv for a while). For me it felt like I was on a fairly good level of English and the only thing I needed was to absorb as many new words as possible. Like in English, at first it's quite difficult and unusual to grasp the meaning, and you need to look up the dictionary (use only Ukr-Ukr btw), but in time it'll quickly get easier. The different aspects of the language just come with it.
I lived in Lviv for a while and for me it was easy, took just a week or less to understand almost everything said without any background in Ukrainian...
I've found my Russian knowledge very useful for doing Polish on here. Much more so than I was expecting.
I guess I would say I'm half fluent in Russian. Then Polish and Ukrainian I can usually guess what people are talking about even though I don't understand every word.
A caveat might be appropriate here: my student friends from Ukraine and my student friends from Russia sometimes do not find it easy to be friends with one another. Even though I esteem both languages as important and even beautiful, I am an outsider to the political realities in these countries. My point is this: I must be sensitive to the full expressions of both sides, and perhaps the best way to do this is to respect the native speakers and allow them to set the rules for their own languages.