This is all my opinion so don't get too crazy please, not gonna edit it just gonna ramble a bit and hopefully thatll be helpful tongue.
Think about the difference in the construction for a second. The circulator design has all the slits at the same level, so each has nearly equal potential at any point during the hit for the vapor to pass through. The slits are also vertical and the vapor is forced through the top part of slit as much as possible so the bubbles are small. Because the slits on the circ are farther out from the stem, the bubbles dont collide with glass or each other as often and stack more. The end result is a very uniform bubbly hit (once you displace the initial water) as more slits operate the harder that you pull/drag, that still has enough airflow for fast clears.
The tree has a slightly different construction, with 2 rows of slits (or grids) and often open ends on the bottom of the tree. This means that when you pull softly, the real action is taking place in the top row of slits/grids, and as you pull harder the vapor is forced further down the trees and out the other slits/grids, and sometimes coming out the bottom ends (and out of everything when you clear it). So the more you pull the more water is displaced as the bubbles form deeper in the water, where in the circ all the water is displaced in the beginning of the hit and as you pull your just pulling bubbles through the water. There are also more collisions of bubbles with the glass and each other so you get larger bubbles forming out of several smaller ones which dont stack as high.
Gridded trees make smaller bubbles so there is more initial diffusion, but there are still many collisions of the bubbles so they don't stack as high as circs. A good mix of both worlds.
Smaller bubbles mean more diffusion, but high stacks of bubbles dont always give the vapor time to condense. Bubble stacks preserve flavor more, and trees take slightly less overall effort to operate. Then again if the vapor is thick coming in it doesnt matter how its diffused youll get thick vapor coming out brow.
My tree feels more 'normal' to me whatever that means in the glass world, and I have much more experience using them than circs, but I love using circs and think they hit amazingly smooth.2c