Results may vary... For many user especially the foam is destroying their hairline, and mostly they switch back to the liquid because they're afraid the lost hair won't grow back...
I'd start with liquid, if having success, stay on it.
No. Its a fairly complicated issue and there are lots of theories as to which one is superior. Regardless of what you read on here, the results of either liquid or foam are completely subjective and results vary from person to person.
The only way you can truly find out is to try both for yourself (for at least 6 months) and then draw your own conclusions.