I have 12, 6, and 4” Robust comfort tool rests. The 12 is used most, especially 8” and up bowls. The 12” is more functional than the 9” IMO. Vibration due to cutting near the end of the 12” is not much of an issue, but does occur. Many ways to address it, but usually s finer cut with a sharper tool does it. I also have the inside and outside Robust bowl rests. Just depends on the curve whether I use them or not. The 6” gets used on smaller bowls, and most of the time cleaning up the bottom of bowls, as well as other smaller stuff. The 4” only gets used on small spindle stuff.
Seldom is the height of the comfort rest any issue with the way Robust tapers the end, particularly with 8” and larger bowls. I find having room to run my fingers/hand under the tool on that curve beneficial. I tried a low profile and didnt like the lack of room.
Whatever you choose, get rests that have hardened steel rods for the tool to rest on. I have a few cast iron and mild steel rests that I dont use, replaced them with hardened rod type rests. Advanced Lathe Tools makes nice rests as well, but I dont have any. Their rod is 3/8” dia, and robust is 1/4”.