Yeah, I could never figure out why but the 18x20 just produces a better ball for me. More penetration and more importantly less balls coming back
Seems to me like you figured it out pretty well.
In my experience...
The UP 16x19, due to a slightly more open pattern, produces more upward bounce off-the-ground, which results in balls that sit up more but have less through-the-court penetration. The result of all this, is that the opponent has more time to get to the ball, as it's sitting up and not shooting through the court. Therefore, players of a better level end up returning more balls back... and it becomes more of a slog-fest than with the UP/UT 18x20.
Both of my UP 16x19 and UP/UT 18x20 are of a similar weight/modification (353g strung, around 6-8 pts HL, and SW around 335-340-ish as an estimate). At those specs, the UP/UT 18x20 has the heft needed to produce a weightier ball, and combined with the lower-bouncing (relatively), more through-the-court bounce... results in a much more effective (at least for me), more damaging shot. The ball stays lower, shoots through the court, and therefore, gives the opponent less time before a double bounce or before it has gone past them. The UP 16x19, on the other hand, feels like much more effort and RHS is needed to produce the same through-the-court penetration... which results in either many more balls coming back... or it being much more tiring/effort or energy-sapping per point.
On further reflection... maybe the "solution" to the UP 16x19 conundrum is a totally different set-up... and/or it suits a totally different type of player. Maybe, what is needed is an extremely polarised set-up where much more weight is added to the tip than the 3 & 9 positions (compared to the UP/UT 18x20) and some weight in the butt cap. This would result in a lower static weight/higher SW set-up... but still more HL balance than a Blade 98 or a Blade Pro. This may then have enough power (due to more weight in the tip than the UP/UT 18x20 set-up), but be lighter and therefore suited to the type of player that wants to use massive RHS on every shot (swing out-of-their-boots on everything). This may then produce more penetration, due to huge swings on every shot, but have enough weight to also keep the opponent pinned back. But this requires a different style/stroke than what I have... and means I have to change my game completely. I don't see why I would do that... when I can keep my stroke speed/natural style of play and have a better impact on my opponent with the UP/UT 18x20 than the UP 16x19.
Hope that made sense... and helped.