I'm a UI designer who works on web apps during the day and Mac/iOS apps at night, and, in my experience, designing APPS compared to designing WEBSITES makes you far more marketable. And you make a ton more money. I don't do much iOS UI work nowadays (I work on my own projects) but my rate of $150/hr was comparable with my friends who do app UI work. Compare that to web design hourly rates and there's really no comparison.
I think the bottom line is if you can tie your work to the actual bottom line of the company (more people signing up and buying the app) compared to just building marketing websites, you're always better off. The key is making sure the higher ups at the company know how big of an impact you make as a designer so they see you as a money-making asset rather than just another sunk salary cost.
I do UI engineering as well. Wireframes/UX diagrams -> high-fidelity mockups -> full implementation of the UI (Cocoa/Obj-C or HTML/CSS/JS). It's nice to do both ends as I can think about implementation as I design and start thinking about where hiccups might occur.
I think the bottom line is if you can tie your work to the actual bottom line of the company (more people signing up and buying the app) compared to just building marketing websites, you're always better off. The key is making sure the higher ups at the company know how big of an impact you make as a designer so they see you as a money-making asset rather than just another sunk salary cost.