Why choose, if you can have both? Today CloudFlare is introducing HTTP/2 support for all customers using SSL/TLS connections, while still supporting SPDY. There is no need to make a decision between SPDY or HTTP/2. Both are automatically there for you and your customers.
Enabling HTTP/2

If you are a customer on the Free or Pro plan, there is no need to do anything at all. Both SPDY and HTTP/2 are already enabled for you. With this improvement, your website’s audience will always use the fastest protocol version when accessing your site over TLS/SSL.

https://blog.cloudflare.com/introducing-http2/