6Al/4V titanium tubing is stiffer than 3Al/2.5V titanium tubing. That's a good point. However, 6Al/4V titanium tubing is not really made from tubes. They are actually 6Al/4V titanium sheets welded into tubes, according to Litespeed customer service. Thanks to Collin Bishops. Even so, they claim that the 6Al/4V is very durable, that's the reason why they provide the lifetime warranty.
Other than stiffness, 6Al/4V titanium tubing costs more than the standard 3Al/2.5V titanium tubing. The reason is that it is harder to weld. Also, this leads to difficult repairing of frames. Once a frame is made out of 6Al/4V, it is a lot more difficult to fix it if the angle or length is wrong or whatever else.
My conclusion is that there is not really anything to compare. A heavier rider would prefer to have a 6Al/4V as it is stiffer.
Here, I also have a question. I weigh 50kg. Would 6Al/4V titanium tubing be much too stiff for me? Would it be too punishing for me on long rides?