[Faterson]

I gave away all my hardware Kindle devices after Marvin for iPad (now also for iPhone) came out last year. To reduce eye strain, the ability to fully customise your font type, font colour and background colour is crucial. Marvin gives you this customisability (the iBooks and Kindle apps do not).

There is no doubt: a traditional Kindle e-ink reader causes lower eye strain than an iPad in daytime. As soon as it gets dark (this could also be dusk/early evening), however, I'd say that the iPad wins: because you can switch to a "dark" theme, with light-coloured fonts on a dark background -- which is especially effective at night when you are in an otherwise completely dark room.

PS: The reason I switched to Marvin almost instantly was its superior annotation functionality -- Kindle/iBooks offer something like 5% of the related functionality compared to Marvin.