As far as which is the bigger bang for the buck depends on how you play and what your fields allows. If you fields allow any firing modes than any of the guns you specified should easily shoot above 12 bps. If you are limited to semi then the Etek/etha, or invasion will allow you to shoot faster easier.
For efficiency, I can get about 10-11 pods off a 68/4500 with my invasion. And accuracy comes from a good barrel, good paint, and a clean properly functioning gun.
If you are going to start playing speedball more than once in a while, then the Etek/etha, or invasion will allow you to play it easier, simple due to the size and shape of the guns. There is no law saying you can't use a millsim gun on the speedball field, but it may make it harder to be truly competitive when you start playing it.
I have always like the looks of millsim guns, but I have never owned because the feel to big to play comfortably with all day.