Rain Kingdra needs a setup: rain.
CritDra needs a setup: Focus Energy.
So, because I wanted a OHKOing monster, I use a Spiker Shuckle, and I let Kingdra give himself his Focus Energy, but I realised that I could be way more efficient if I had a Baton Passer to give me the Focus Energy (btw, I still don't know if I can BP that), and by looking at the possibilities, I saw Blaziken with Speed Boost. But since he's Uber, I thought that I might have to use Combusken (which is probably my only possible choice now, and I still don't know if I can Baton Pass Focus Energy and Speed Boost without breaking the OU BP clause).
So, when I saw Speed Boost, I knew I could combine both Kingdra sets, but in a better way (since rain doesn't last forever).
And, you say Rain Kingdra is better because it's faster, yet you forget the:
-Priority status
-Priority moves
-Very specially bulky pokemon like chansey. It does even less damage to them. 2.25x less to be exact.
I'm sorry if I'm just being really dumb, but I don't really see how one set could be better than the other, because if you're faster, you might not get the KO, and get KO yourself, or, if you're stronger and OHKO the ennemy, you might be slower and get hit first (that's why I use a Shuckle buddy here)