I would definitely NOT start with a straight line ruler, unless you just want to practice, because straight lines are most easily doable with a walking foot as you already do. The straight line rulers were designed for longarm quilters that don't have a feed mechanism. They were downscaled to DSM's but I think they aren't needed there.
I would get something with a slight curve or clamshell. Or, get one of the spirograph type rulers if you can find a design you think you would use a bunch. Choosing a ruler is really more about how YOU would use a ruler than anything else. It depends on what you're trying to do.