What is the benfits of either ? When is "with v/a" better and when would you choose the one without v/a ?

Vacuum advance for street cars, no vacuum advance for race cars.

Ok ! But why ?

Ok ! But why ?
Vacum advance adds more advance at part throttle and cruise. This helps with mpg and emissions. A engine makes the most power when the timing is advanced just below the detonation. Vacum advance needs to be tuned to your combo, as in how much advance to add and how much vacuum is needed to start adding. A lot of this depends on your combo. Vacum advance doesn't work at full throttle (vacuum drops to zero) so it is not needed on race only cars.

Vacuum advance units are for emission and fuel economy only has no effect on performance at full throtle, check out this article it will explain how and why also whitch pn to use and how to tune it to your setup.

http://corvetteforum.com/techtips/viewsubtopic.php?SubTopicID=115&TopicID=3