Basically, no e-ink device is going to have a really great refresh rate. You will have to choose between reading outside and responsiveness.
If I were you, I'd pick responsiveness, and get the iPad 3. If you're concerned about storage, try using Dropbox or other could storage. The ecosystem IMHO doesn't really matter for you, since you're not thinking about buying from iBooks, so you won't be locked in anyway. Reading apps don't cost that much, so if you want to switch to Android or Windows on Arm in the future, it won't be that difficult. The reason you should get iPad 3 is that you care about resolution, and IIRC the high res Android tablets coming out are going to be more expensive, and also for reading documents, 4:3 is the ideal format, and there don't seem to be any 4:3 high-res Android tablets.
I have one of the Tegra 2 Android tablets, and it works fine for reading docs, but the 16:9 screen ratio just is suboptimal for doc reading.