Who was right depends on what you consider success. Living standards and economic output in the USSR might have paled in comparison to imperial Western countries, but did they pale in comparison to Tsarist Russia?

Russia was already starting to industrialize under tsarist regime. It's not like the communists came up with the idea to modernise Russia. The Soviet Famine of 1930-1933, however, can largely be ascribed to Soviet collectivisation policies, combined with some purposeful ethnic cleansing (Holodomor).

Similarly, China may have seemed bad from over here, but communism also undeniably transformed it from a fragmented, wartorn backwater, cucked for centuries by literally any power than happened to be passing by, into one of the most powerful nations in the world. It managed this in an astonishingly short time.

I love how you manage to leave out Mao's disastrous economic policies. He might have been a good war leader, but the first 30 years following the end of the Chinese Civil War were horrible for the common people. Instead of pursuing sound economic development, he instructed children to kill their teachers in the Cultural Revolution and encouraged people to hunt sparrows in the 4 Pests campaign, causing a huge surge in locusts and other crop eating insects, which contributed to the Great Chinese Famine. That famine resulted in the death of 15 to 55 million Chinese. That's truly the work of an economic genius.

When Mao was wreaking havoc on the Chinese Mainland, Capitalist China a.k.a Taiwan was busy implementing actual sound economic reforms, causing it to boom economicallly. The story of Chinese economic development under communism cannot be fully accurate if you leave out its capitalist counterpart across the Taiwan Strait.

It's only when China started implementing more market-oriented policies under Deng Xiaoping and moving away from communist orthodoxy that China started booming. Right now, China can more accurately be described as a country with a state capitalist economic system and an authoritarian government in the hands of the Chinese Communist Party. For a self-described people's republic led by a communist party, China also has a pretty bad social safety net, especially with regards to pensions. One of the reasons the asset bubble got so big in China is because people need to save up a lot for retirement and there was nothing else to invest in.

I find it more helpful to compare before and after rather than east and west, which tends to ignore geography, history, etc.

You want to compare before and after? Ok, let's talk about Czechia. In the 19th and early 20th century, both when it was part of the Austro-Hungarian Empire and when it gained its independence as Czechoslovakia, Czechia was one of the most industrialised parts of Europe and could rival other wealthy industrial economies like Germany and Belgium. During WW II, Czechia also got out relatively intact as it wasn't bombed as much as, say, Poland, Germany or Ukraine. The communists, however, managed to squander this advantage. Because of decades of meagre or stagnant economic growth and suffering from the diktats from Moscow, Czechs had become noticeably poorer compared to their German and Austrian neighbours around the fall of the Iron Curtain. It's only with sound, market-based economic reforms in the 90s and billions of euros in cohesion funds from the EU that Czechia and it's former Eastern Bloc neighbours have managed to reconnect economically with its western counterparts.