Every dragon can grant help completing an Episode Battle. Choosing that wish will give you an exclusive item called a Dragon Orb. These can be used only when hovering over an Episode Battle with branching paths, as they make the secondary objectives easier to meet. Even though a lot of these objectives just require you to win within a certain time, using a Dragon Orb for a bit more leeway can be a big help. The only caveat is that you won’t earn victory rewards from it, but that’s not too bad of a trade-off to see some really fun what-if stories.
This option won’t pop up until you’ve come across an Episode Battle with potential branching paths. Although that occurs early on in Goku’s story, those who immediately spent a lot of time in multiplayer modes or received a free Shenron summon—depending upon which edition they purchased—would likely not have seen this. Make sure to keep an eye out for this option if you didn’t see it during your first summon(s)!
Dragon Ball: Sparking Zero doesn’t spend much time explaining the nuances of summoning the dragons, and leaves it all for us to discover. As fun as that idea is, nobody likes to feel like they made the wrong choice, especially with how rare it can be to see Porunga and Super Shenron.
The best choice for you depends entirely on preference and how far along you are, but you should be better equipped to select a more favorable option. Go out and make your wishes in confidence!