iBooks might be limited to iOS but ePub itself is supported by a lot of platforms. Unless you need bookmarks and location syncing, using iBooks on iOS and mayhaps Aldiko, Mantano, Moon+, etc on Android works just as well.True. I use Calibre to convert then load.
iBooks isn't bad but I am a multi OS person and find it personally too limiting.
Personally, I only use the Kindle app for Amazon-purchased ebooks (and only when I'm desperate). I hold my iPad with just one hand and tend to switch between hands when getting fatigued. The ability to customize controls so single-tap on both the left and right margins goes to the next page is a must for me. On the Kindle Paperwhite, you can't customize controls but the left tap region is much smaller than the right so it's easy enough to stretch the left thumb to advance to next page.
Aside from the times I read newly purchased ebooks on the Paperwhite, I "sanitize" all my purchased ebooks and convert them to ePub with Calibre for loading on Marvin (iOS) or Android.