[AnemicOak]

I currently use an iPad Mini along with Calibre Companion for library management and MapleRead for reading. In the past I've used various Android based tabs with CC and Bookari and Moon+ Pro (among others). Both platforms have apps that will handle various file formats, but I tend to prefer to convert to ePub and stick with my favored app. The exception to that is PDF which I prefer to not mess around with trying to convert and instead leave as a PDF and read that.

As to iOS vs Android I find both work equally fine for me. For myself screen quality is going to be at the top of the list when looking for a tablet that will primarily used as a reader. While it's not necessary to have a super high PPI screen I find I tend to prefer one that'd fairly high in PPI and one where I can set the screen brightness fairly low (there are also apps to help drop brightness below what the tablets controls do if necessary). Both Apple and Samsung, among others, make very nice solid devices. For me devices like the iPad Mini and Samsung Galaxy Tab S2 8" are a great size.

As Cin mentioned a tablet is not going to be the number one ideal choice for sun/bright light. I find that 95% of the time with the proper brightness setting I can read quite comfortably when outdoors, but at the cost of much faster battery drain (due to cranking the brightness up for a sunny setting). For a setting such as a nice sunny day at the beach I'd generally prefer to grab an eInk based device.