The simple answer is yes, Losmandy is better.
It's a pretty simple engineering formulea (high school engineering studies) to plug turning moment and material flexure for Aluminium.
http://metalab.uniten.edu.my/~Halina/EXP3~1.pdf
But even forgetting design consideration - the Losmandy D saddle cross bar is the same length but considerably thicker than the Vixen cross bar.
Assume its the same material, same length, same load and flexure is linear. So a crude estimate is under the same load if the Losmandy bar weights 4 times more than the Vixen one - it's because it's four times thicker, so its flexure will be four times less!
Shape will slightly alter this - but as a reasonable guide - weight you bars and the difference in weights I would expect to be the difference in flexure.
Finally if the Losmandy bar is 1/2" thick by 4 inches wide and carries say 10kgs of gear, placed horizontally (so maximum stress when the scope is parallel to the ground) and bar secured top and bottom of the saddle - plug that in with a flexure under load given a coefficient of elasticity and you will get exactly how much elastic flexure will occur!