I haven't taken the official Gmat yet, so it is hard to tell the difference between Gmatclub test with official one.
I just took the test in Gmatprep software, and from my exprience, the difficulty between Gmatprep and Gmatclub is so close.
My first test in Gmat was the free one on Gmatclub, the result was 600. 1 month later, after finishing the Manhattan SC, I took test 1 in Gmatprep and scored 630.
The Quant test in Gmatclub is my favour. So far, I finished 4 quant tests. The results are 47, 48, 48, 49. Seeing the score improve is excited. After each test, I always analyze my fault and draw myself experience. I am really interested in Best/worst section where I can easily realize my weakness and thus, have more practice in those fields. The timing table appears after each test is also great. It helps me adjust time generally.
I don't have much experience with the Verbal test since I had only 2 tests. One thing troubled me much when the second test I took overlapped some questions I had done in the first test (although the question bank is still abundant)