Nice idea for a thread.
I tend to prefer medium to larger trees myself, but for a very practical reason. I prefer natural looking trees, and the larger size gives me more room to work with. For me, smaller trees are like deadwood features - I can absolutely appreciate them when they work, but when they don't they tend to ruin the sense of scale for me.
But honestly, the same goes for the large ones too. Large for the sake of large doesn't work for me either. That larger one you showed as an example is good long-term raw material, but that little one you used as an example is incredible. In their current state, the little one wins hands-down. But it might be difficult to keep that little one looking that good over time whereas you probably would have a lot more long-term options with the larger one. In it's current state, though, I'm not much of a fan of the big one. Not only does the canopy look like a topiary, but I don't really like the deadwood either. So that's two strikes against it in it's current form.
The one in my profile photo is a little one too. It's a seiju that fits in the palm of my hand, and it's probably one of my more realistic looking trees. I love that one. It's also one of the smallest trees I own, and definitely an outlier next to most of my other ones. Most of my trees are probably in the 12-36" range, and although they tend to be much longer term projects, I feel I can make them look more like actual trees I see in nature that way. For me, it's all about working towards a realistic sense of scale.
In a show/demo/bonsai garden setting, my eye does tend to gravitate towards the larger ones, but for the same reasons. The best large trees tend to look more natural to me than the best small ones. When done well, the level of detail in a highly refined large tree is pretty difficult to match in a small one. So for me, I guess it's mostly practical based on my style preference. But I love it when I find a great small tree where I can lose myself in the details. I just find those to be a lot more rare.