Honestly, if I got a cube, I would go for a 6 or 8, as for me, cutting accuracy is harder with smaller pieces while at the same time, spot on cutting is all the more important with smaller pieces, so the die cutter would be helping me work with stash fabrics and make patterns that I would otherwise tend to avoid.
I have a good collection of rulers and I am competent at cutting large shapes with reasonable accuracy, so while I might like and use bigger die shapes, not having them is not keeping me from making anything.
Everyone's preferences and gaps in what they have will vary.
Another criteria would be to think about what you envision yourself making. Quick baby quilts, graduation gifts, and donation items? Then quick and big blocks are the way to go for sure. IF you think you might like to make an heirloom piece, are drawn to pillows, table runners, bags, and the like, then maybe the smaller blocks will be more useful