One of the fascinating things about AI and cryptography is how similar the math sometimes is. Like, the way that the poseidon hash works (matrix mult -> per-unit nonlinear layer -> matrix mult -> per-unit nonlinear layer) is *exactly* what deep neural networks do.

If you were again 15 today, work on AI or Eth?

AI would always have happened. The theoretical aspect has been worked out for decades but computation and data is a bottleneck. 

The creativity involved there is not even close to the creativity required to create Ethereum. Vitalik is a pioneer.

AI. I'm 39 and reskilling to work in AI. 
I would say that, in the near term, say the next 10 years, AI will be more important. In the medium term, it will be blockchain, and long-term it will be AI again