I'm not sure this will help you pick a device but, depending on which smartphone you have you might be able to stop worrying about transferring via micro SD card. Most phones, not all, can handle USB2Go and you can carry a short cable with you and transfer them with that. I think my shortest USB2Go cable is 4" long. I'll be surprised if there aren't shorter ones. It's bigger than a micro SD card but not as easily lost.
I've transferred books a couple of times from my phone to my Kindle with a cable.
Barry
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