The reading experience for Kindle, Kobo andiBooks apps are all very similar. As you have the freedom to use whatever 'reader' you want there is nothing stopping you using as many as you want. The benefit of this is that there is often a discrepancy of the price for the sa,e book on the different sites - so you buy the book at the best price. In addition they also run offers and sales from time to time which again can save some money.
IBooks is the most seamless to use as its the only app that allows you to order and download books from within the app. Kindle and Kobo require you to create and account on their websites and order books from there. This is not their fault - its down to restrictions on apps by Apple.