I’m in a similar situation, where I have two game ideas that I want to work on, and one is a small, simple game that would probably be well suited to mobile, while the other would be a huge, ambitious project. I’m still learning the engine, so I decided to work on the small game so I can get my hands dirty with it and get comfortable working with the engine before I try tackling the bigger project.
I’m pretty excited about both game ideas, though, so it’s not an issue of choosing something easy over choosing something I’m passionate about (the small game is something easy that I’m ALSO passionate about :D). If you’re really passionate about making your bigger project and not so much about the small one, my choice might not necessarily be right for you. In the end, I think you have to work on what excites you. If you’re not excited about a game you’re making, you’ll probably have a hard time finishing it.