There's a night and day difference between the Greddy and the Top Speed manifold. The exterior casting is identical (besides where the Greddy logo goes), but what makes the difference is the internal portion of the manifold. If you have both sitting side by side, you will see the and feel the difference between the two. The Greddy is smoother compared to the rough surface of the Top Speed.

As for cracking, I personally have not heard any of them doing so. I personally have always gone with the Greddy manifold and have compared both.

From threads and posts in the past, the Greddy manifold will only really make power on high horse power motors at higher RPM (please correct me if I am wrong); therefore, it would be interesting to see what the differences would be on the two manifolds on a dyno run test.