I have bought a number of Otto's Boxes in the past, and if offered again I would likely buy several more. But I won't be buying any Bigby's Boxes.
The TR process for me is fun initially, but becomes unfun in the later levels. The early stuff is fun, there are a variety of quests that are forgiving for whatever playstyle I choose for that life, and the XP requirements for leveling aren't onerous. But in the later levels, 16-20 or so, it becomes an enormous grind. There are far fewer quests to choose from, it can be difficult to keep an elite streak going when playing solo, and the XP required per level is ballooning exponentially while many of the quests in that range give absolutely terrible XP.
To me, the entire purpose for buying and using an XP stone from an Otto's Box is to mitigate that grind from 16 to 20. That is the motivating purpose to me, while the 8-16 part of the stone is merely a minor bonus. I don't care that much about skipping 8-16; XP is still plentiful in that range, there are a lot of fun quests to choose from, and the grind isn't too bad either.
The printed description of what the stones do is 100% irrelevant. All that maters is how they actually function. Otto's Box mitigates the grind from 16-20 that I find unfun, thus increasing my enjoyment of the game and making me willing to buy them. Bigby's Box does absolutely nothing for 16-20, so it does not increase my enjoyment and I won't be buying any. It's as simple as that.