Neither are particularly good measures. For instance, businesses are very unlikely to jump on new releases very quickly. If the ratio of corporate owned to individually owned computers changes, it completely skews the data.
In addition, there is a huge number of machines out there that simply don't need to be replaced. They are 'good enough'. It is very natural for sales of PCs to slow down as the market gets saturated. The service life of a PC is much longer now.