OK I found this:
BioWare Breaks Down Mass Effect 3 Game Modes
"All these settings do is that they set some of the options on the option screen before you've played it and know what those options mean. Once you get in and start playing, you can change things," he said. "The story mode is actually just a difficulty setting. The action mode is actually just about choosing 'automatic dialogue'."
It's still possible to die in story mode, it's just quicker and easier to blow through the combat and get back to the serious business of macking on Jack. "You get through it faster. You can mow through enemies," he added. "Generally, if you're trying, you're not going to die."
Action mode, meanwhile, puts players on a "default path" that blends Paragon and Renegade actions, but Hudson was quick to clarify that "default" does not mean canon. "We have a rule in our franchise that there is no canon. You as a player decide what your story is," he said. "But we choose a default path that gives you access to a lot of cool things. It's like how a character like Jack Bauer has to make some decisions where he feels empathy in one moment or feels particularly brutal in another moment. We weave you through a default path that switches between those."
Story/rpg shouldn't have any differences in choices. It's nice that you can change them after starting a game.