The cooler setup is simpler (less gear), so it's a good start. That's why I thought I'd still use it even after I set up my RIMS. Didn't turn out to be the case though...I always end up reaching for my RIMS gear. Takes a bit more effort to set it up, but once it's going...it's easier to hit your temps, easier to get a clean wort, easier to step mash, etc.
So, I'd say get the cooler, you'll always have use for it. But if you feel confident about your desire/ability to jump right into RIMS, there's no reason to hold back. Once you get it up and running, it can make your life easier.