Berlin is considered to be very hot right now. A lot of start-ups, especially those operating in the DACH region (Germany, Austria, Switzerland) are based there. Compared to London, life is fairly cheap and from what I've heard, there's great access to talent. From what I know, there are many working spaces and in terms of quality of life, Berlin has a great reputation.
In terms of access to funding, I would say that London has more opportunities, and that they are more organised.
One thing you should also consider are your target clients. I don't know what kind of product you have, but in my experience, being able to meet potential clients in person makes things a lot easier.
Depending on your business, you may have clients all over the place and may need to travel. In that case, you should also consider airport proximity and cost of travel.