If you only need your device for reading, wouldn't an e-ink device be better? They have amazing battery life and clear crispy screens, though no colour of course. I can read manga just fine on my prs-505 and my cybook. The 6 inch screen is only a little smaller than a normal manga page, so no problems with legibility and no need for zooming (at least for my eyes). You might also consider an e-ink device with larger screen, but of course they are more expensive.
Do you want to read specifically mobipocket ebooks or can you convert them? Not many devices support those any more. Certainly the sony doesn't. The cybook does, and I think the kindle.