If your deciding between the kindle and Nexus I would go with the Nexus personally. With the kindle I believe you are locked into only being able to buy from the kindle store opposed to on the Nexus you can use both Kindle store and Nook store. As far as collected editions go yes their are books you can buy as the collected edition rather than single issues. Usually results in being cheaper. If you haven't check out
https://comicstore.marvel.com/collections that is the list of collected books you can buy for the marvel app. The subscription I'm not sure about. I know their is a marvel subscription but you have to read the comics through a web browser. At least that is what I have read. Also I'm sure you know about
www.comixology.com but if you haven't it is a great site for digital comics. If you have any questions about the Nexus let me know and I am sure somebody in this thread would be willing to answer stuff about the kindle as well. Honestly either way you can't go wrong.