3/2.5 is easier to work worth, Scooper...further the aerospace and military industry has been gobbling up most all of the 6/4 supply there is in the USA. And most builders no longer can source enough for a semi-reasonable price to bother marketing whole framesets anymore. Even the Lynskey Scooper cites is not entirely 6/4, only the front triangle.
Originally Posted by Lynskey
[snip] sheets are rolled into tubing and seam welded together by Lynskey. These newly-formed tubes are then welded together to create the R450’s front triangle. [snip]
6al-4v titanium front triangle for maximum stiffness and power transfer
https://www.lynskeyperformance.com/store/r450.html
Now as with any material, a builder can make any frame so stiff that you will not want to ride it...as it will rattle your fillings out. The question is how much it would weigh. Unless you OP weigh 300+lbs (or badly lie bout your weight to the builders) it would be hard for a reputable framebuilder who knows what they are doing to make a custom Ti frame a noodle. And material would factor in, but in and of itself 3/2.5 does not make a frame noodly. It is what you do with the material.