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The USSR championships and the Candidates tournaments are two of the strongest tournaments in history.
The USSR championship was typically a round robin event (sometimes Swiss), where top Soviet players would compete in fields consisting of former and future world champions.
The Candidates consists of a similar field in a round-robin format.
Which one was the stronger tournament? Which one would have been harder to qualify for?
The USSR championship was actually the usual way for USSR players to qualify for the Interzonal. This was an international tournament, generally featuring a number of the best players in the world, and the best performers would qualify for the Candidates. So since the USSR championship was generally a necessary stepping stone to the Candidates, qualifying for the former would have been easier by definition.
As far as how strong both tournaments were, this is more complicated. To win the USSR championship, you had to do better than the other top USSR players (quite a difficult feat!). To win the candidates, you'd have to beat the top finishers of the Interzonal. The Interzonal included the top finishers in the USSR championship, as well as top players from around the world. So generally, the candidates would have a stronger field.
However, there's also the complicated issue of USSR players sometimes prearranging draws. E.g., for the 1953 Candidates tournament, Bronstein admits decades later that the USSR players were essentially forced to prevent Reshevsky (an American) from winning. This naturally involved giving draws (or perhaps even throwing games) to the USSR player(s) who had the best chance of outscoring Reshevsky.
So in a round-robin tournament with USSR players and outsiders, the outsiders would have a decreased chance of winning, which would naturally raise the chance of a USSR player winning. How much of an effect this had is of course debatable.
Although note that sometimes the Candidates were held as a series of matches. Generally there were 8 participants, and 4 matches to begin with. The 4 winners of these matches would go on to the semifinals, where they'd play one of the other 4. The two winners would face each other in the finals, and the victor would win the candidates. In such a format, the USSR players would have no added advantage from prearranging games, since when they were playing against each other it would be in an isolated match. When Fischer won the candidates in 1971, it being held as a series of matches definitely helped him a lot (maybe he could have won the candidates as a tournament as well, but his odds would have been lowered).
Meanwhile for the USSR championship, it just involved USSR players playing against each other. So, they would have no obvious incentive to work together in order to prevent one particular player from winning. This means that for one of the top USSR players, winning the USSR championship could at times be potentially more difficult than winning a candidates round-robin tournament.
TL;DR: the candidates was harder to qualify for, and in principle was much harder to win. But USSR players potentially fixing games could skew the odds to some degree.
