To give an example, with CM you can choose to build ad hoc reports with either a full data set or a sampled data set. GA will sample data and give you an accuracy rating, but inaccuracies can grow from this depending on how you use the reports (e.g. comparing multiple metrics over a long time period). I have no idea if the premium version of GA gives you access to full data sets but that's even more expensive than CM.
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How is it more accurate? Are they not using javascript? Do they do less sampling for larger properties?
To give an example, with CM you can choose to build ad hoc reports with either a full data set or a sampled data set. GA will sample data and give you an accuracy rating, but inaccuracies can grow from this depending on how you use the reports (e.g. comparing multiple metrics over a long time period). I have no idea if the premium version of GA gives you access to full data sets but that's even more expensive than CM.