Here are a few key differences:

* Our iPhone app ties in with our web app, which allows you to prototype both web and mobile. http://www.marvelapp.com. Our iPhone app is just one part of the eco-system

* We support PSD importing directly into the iPhone app

* All your images are synced with Dropbox so you can easily access them

* Our web app allows you to download prototypes as ZIPs to run offline or on your own server

* You can select dozens of different device frames (HTC One, Lumia, iPhone 5S etc)

* You can export iPad prototypes using our iPhone app

For starters it's not available on Android.

I've been using Pop App for a couple of months, but I downloaded Marvel's app today and love it. Just so much smoother and a lot more features.

No offense, but did you bother to research Marvel _at all_ before asking this question? popapp app's landing page makes it very clear the workflow is physical paper -> photo -> storyboard, whereas Marvel's landing page is very clear their workflow is PSD/etc. -> storyboard.

Sometimes I feel like HN asks "how is this different from X app" without doing any research, just to be lazy.

No offense, I don't know where you did your research but I don't see anywhere on Marvel's landing page where it makes clear its main flow is PSD -> storyboard. In my opinion, Marvel's landing page makes it very clear that workflow is paper -> photo -> storyboard just like Popapp, but also supports importing a PSD image (which doesn't seem like a groundbreaking differentiation). Granted, the founder explained the difference somewhere above but they're still "improvements", they fundamentally do the same thing.