Laying up is almost never the best option if the goal is to shoot the lowest score.
A 100-yard shot is likely not "easier" than a 30-yard shot.
That's not what you're saying, though. In laying up you need to play a 120-yard and a 100-yard shot. In going for it you make one full swing and one from much closer. The comparison is not "short iron versus fairway wood" - it's TWO short irons versus a fairway wood and a short game shot.
It goes without saying (nearly) that the "lay up or go for it" discussion is largely based on the assumption that there's not a lot of trouble like water, etc. around the greens.