Companion cube is easily rarer to get, if you have enough coins for it, and baby skeletron shows that you defeated the Dungeon Guardian. However, it doesn't show whether you did it multiplayer, or someone just gave it to you. I easily think that Companion cube would be better in terms of showing off.
Neither, I'd choose the Eater's Bone cuz the baby eater is absolutely adorable. But tbh Dungeon Guardian cuz it shows that you might have (we know who you are bone key giveaways) beaten a dungeon guardian (like me in solo with a ton of setup)
The baby eater is one of my favorite pets of all, the only one I like more is the black cat (I want that thing so bad) but I suppose the dungeon guardian and the companion cube are alright, I'd still choose the dungeon guardian cuz the cube is just luck based and all it requires you to have is tons of money sooooo