The border changes between the Soviet Union and China between 1917 and 1945 seem to be very complicated. At the end, in 1945, the Soviet Union invaded Manchuria wholeheartedly, and in violation of their neutrality pact with Japan, seizing huge swaths of land. Did the Soviet Union later return this territory to China? What was the net change from 1917 to 1945? Did the Russia/Soviet Union end up even larger by 1945, or did China expand at the expense of the old Russia Empire?
The only documented change of territory that I can find references for is the annexation of Tanu Tuva, which was occupied by the Red Army in 1921 as stablished as an "independent" People's Republic under Soviet control, and formally annexed it in 1944.
Later on, during the Sino-Soviet split which led to the Sino-Soviet war, China made some claims against border agreements with Tzarist Russia (as being a result of unequal treaties), but apparently did not claim any other modern change, which could be a very good indication that (except for Tanu Tuva) the borders were kept at the pre-WWI lines.
I am not including Mongolia here since, while China lost it, its territory (except Tanu Tuva) was not incorporated into the SU (also, arguably it was independent either from 1911 -declaration of independence- or 1915 -recognition by China- even if it was later briefly occupied again by China, so it may fall outside the period being asked for).
Note: Since you could consider that Mongolia was independent before 1917 and Tanu Tuva was carved out of Mongolia, you could also claim that it was not a loss of territory by China but by Mongolia. Anyway, that is just splitting hairs.
Certainly the answer is hard. Maybe we can separate it in two main zones. One is Mongolia and the other one is Dalian (also named Dairen or Port Arthur).
Before 1921 Mongolia was under influence of China, while after that date the influence went to the Soviet Union.
Dairen is a port that was occupied by several countries during XIX and XX centuries. After WWII it was under soviet rule, and they gave it back to China by 1955. But during the period of your question it was under japanese rule, so it was neither russian or chinese.
Therefore, since Mongolia was an independent country after 1921, maybe the answer to your question (until 1945) is favorable to the Soviet Union in the net exchange. While after 1955 China wins against the Soviet Union thanks to the recovery of Dalian.