Both Vacumatic and Aerometric 51s would write the same. It is, mostly, the filling system that make people prefer one over the other. The diaphragm on the Vac 51s do fail more frequently (5 to 15 years) than the sac on the Aeros. And the replacement of the diaphragm is a tricky job that not every one is able to accomplish at home. In most cases one has to send it to a pro and, thus, pay for the repair cost upto $25-$30. On the other hand, almost 99% of the Aeros are found with the original sac that the pen came out of factory with some 60-70 years ago still in good shape.
Even then, there are peoples who prefer the Vacs over the Aeros for one reason or the other (apart from the filling system) such as colors, double jewels (on some of the Vacs), ink capacity or variety of cap / clip combinations.
Edited by mitto, 20 November 2017 - 11:29.