These days it makes no difference whatsoever. The angle between the cylinders has no bearing anyway on performance or power delivery. The only thing it influences is primary balance. Ducati use their fairly dated 90 degree twin layout as each bit of reciprocating mass cancels eachother out, although secondary vibration is left unchecked. Everyone else has joined the twenty first century and uses counterbalances to kill vibration, or tame it to the chosen level. The angle between cylinders being a compromise between the overall length of the engine and getting enough meat between the cylinders. The only reall difference is the firing order. Which is why I rather favour the parallel twin. All the performance charactistics of the allegedly superior v-twin, but alot more compact, easier to service and cheaper to build.
Or something like that.
