2 into 1 technology is the best choice for most applications .
The VE of most engines is more realized with the two cylinders ( or more )combined.
That all said , size, length does matter.
The vendor should be happy to show you real dyno reports under various conditions.
Dont fall for peak HP BS
Look at the Torq at RPM you most likely operate in .
To fully achieve optimums either a dyno (esp one that has a GOOD dyno opertor) or a wide band oxy sensor AFR gauge will help.