Which would be better, steel/graphite shafts?
Well, based on what you have described about your swing, I would say graphite. Although the difference between graphite and steel in irons is not that much since it is a short club, you will get a little more flex and lighter feel from the graphite. Also, the x-18s are slightly heavier than the DCIs so the weight in the two will most likely balance out. So to me, it comes down to shaft flex, in which you will get more with the graphite = more distance = lower scores (hopefully!!). So my advice, get the x-18s, good clubs and better option of the two, GL!
If you’re a beginner, Titleist isn’t the set for you. Either set really. As far as shaft options, you can get a uniflex steel shaft in your irons since you don’t have the clubhead speed as of yet. I’m not a fan of graphite shafts for irons. You want control of your iron shots and IMO, you have to have a feel for the shot in order to control it. I have more feel with steel shafts in your irons. You will too!