In my personal experience, python is great for prototyping, but you'll be in growth pain, sometimes even as early as a hundred active users.
On the other hand, for me go is almost as productive as python. If i work in a team, i'd even say go is more productive. The simplicity and straightforwardness of go really saves a lot of trouble.
My personal preference of the two is Go, but i'd only use it if the code itself is important part for the product.
If the goal is to develop something for the web quickly, i'd use neither. They both have very basic UI features. Something like PHP or nodejs will give you results much quicker.
A big lesson for me was that quickly delivered questionable-quality software is usually more successful than great software with slightly slower development cycle. Go was not the right choice for the former.