jawjaw pretty much covers it, but here's some more intel.
SJ, Nero, DTD are the ones where you can make a decision.
SJ - one shot. So its terrible for mb. I don't even like to play SJ during a mb in the off chance that I start the Galactic Mode by accident.
Nero - great for mb, but scary for regular play.
DTD - Gives you Galactic Spinner, which is 75k per spin. Definitely worth good points, however, you can get the spinner value up to 68K via the TREK lanes. If you've got it maxed out, you don't get to much more for the Galactic. If you've got that away team shot down, then hey, why not. The flip of this means that it's very valuable to start Galactic Spinner if you're early into your game.
Some additional spinner intel.
Spinner increases every time you spell TREK.
Spinner resets every ball. However, once you get it to 20k, it resets to 20k. Meaning on your first ball it benefits you to pay extra attention to the TREK lanes and get it there.