This is based primarily on the device themselves, the iPad was always a multi-function tool which could be used in many ways with the price tag to match. The ‘True’ eBook readers on the other hand when first introduced looked nothing more than glorified calculators – Grey scale screen!
There was a time when readers were expressing dis-taste towards the glossy iPad screen, not so anymore, also we see the full color eBook readers from Barnes and Noble’s Nook stable (not just for horses) so color is now a good thing.
The aforementioned trend is that readers on Kindle and Nook et al. are more likely to engage in the Fiction titles out there, making up 20% of downloads according to research by the New York’s Book Industry Study Group. Whilst the iPad readers are more likely to download the interactive books that contain multimedia or charts, no surprises there really but this will help in choosing file formats or strategy generally
With the safe option probably being to prepare publications which cater for both devices .mobi and .epub files alike so your readers will always be taken care of regardless of device.