So, I was just going to use the DC app because honestly I'm only really interested in Gotham-centric tales. I tried the comixology app and holy shit! Not only did my hand full of comics transfer, since both apps are powered by comixology, but I can get ANY comic or graphic novel with this app for a fraction of the news stand price. So exciting. Quick, someone give me a job.
Yeah, the DC and Marvel apps are just branded versions of Comixology. Everything you buy transfers as long as you sync your account with the Web site.
You just missed all of the big Batman sales that coincided with the movie...RIDICULOUS. Everything half price. Be vigilant, their .99 cent sales are worth waiting for. DC doesn't have set sales, but Marvel does a Friday one and a Monday one.
Worst was the Grant Morrison sale they did. I blew like fifty bucks, but bought $100 worth of comics.
You should check out Gotham Central, it's an often overlooked Batman title with a cool focus on GCPD cops.