I use mechanical secondaries with a 4 speed light car cuz I like the secondaries to come on when I want them and don't want to have to wait for the vacuum signal to open them.... Just the old circle burner/throttle control stuff coming out in me, I guess..

Carb selection would also depend on your driving style, I like going fast and good handling. Both seem to be easier to achieve with the manual trans/mechanical secondary combination.... If your just going cruisin' and travelling, the vacuum secondary will do just fine....