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Ok central obstruction does give more contrast to viewing the planets with a Newtonian.
However, things you need to ensure performance on an SCT is as follows.
1: the scope must be at ambient. SCT's often take 4-6 hours to cool down to ambient. There are several methods for cooling SCT's down.
2. Seeing is king. If you don't know what seeing is, then it is a sure sign that you are not collimating at the highest level possible for an SCT, nor are you able to see anything due to poor seeing. Only view the planets during periods of good-great seeing. The poorer contrast of an SCT needs good seeing, otherwise the view will appear to be worse than a Newtonian.
3. If you are not already doing it, collimate your scope regularly. SCT's have collimation shifts even without being moved off the mount. The primary can get slop in it and that can cause collimation errors. Only collimate in good seeing.
4. You can help contrast on an SCT by flocking it well. It will not give you as good a contrast as a Newtonian but it can improve the performance.
5. Use very high quality eyepieces. Good quality optics make a difference with viewing the planets. Barlows and Powermates also impact on detail being seen.
Besides all this, you cannot really compare both types of scopes. Each has its strengths and each has is weaknesses. Just a fraction of miscollimation on an SCT makes a huge difference.
Hope this all helps.
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