Part of the inconsistency of our policies with China and Cuba stem from concerns more practical/realistic than ideological.
Sure, to have an ideologically consistent stand against Communism, we should (theoretically) enforce a trade embargo against both countries. However, there are differences.
First of all, Cuba is so much closer to the U.S, and thus right in our backyard, as it were. More incentive to keep them in line.
Second, and perhaps most important, China is huge and has the potential for a world-leading economy. I believe the U.S. feels this is inevitable, and wants to establish the best relations possible to get in on the action and also to get in on the ground floor of economic influence. I personally don’t think China cares one whit what we have to say about their human rights (or other) policies, but extending trade relations as we have at least gives us somewhat better standing.
Expanding on this; I think the U.S. hoped a trade embagro would cripple Cuba and force Castro out either by insurrection of the people or desperation of the Cuban government to make its own changes. No way in Hell this would ever play out in China regardless of U.S. policy.
Also, Cuba always had the backing of the USSR to help pick up the slack from the trade embargo. I don’t think the USSR would have given China a bowl of maggoty rice regardless. So, in effect, our stance against Cuba could be seen in part as a proxy protest against the USSR and its influence 90 miles off our coast. No such effect for our relations with China.
Of course, now that the USSR is no longer, the Cuba dynamic is somewhat changed. So, the case can certainly be made for a change in trade policy with them. It is not an easy situation and direct comparison between our relations with Cuba and China is difficult.
And I still think the U.S. is holding onto the embargo to force Castro out, returning Cuba to some semblance of a democracy. Maybe if we thought we could accomplish this with China, we would have a similar policy. But that ain’t gonna happen.