I think on balance, outcomes will be consistently better if you reversed the order (unless significantly overweighted, but there should be redundant buoyancy anyhow either drysuit. extra bladder).The protocol I use/teach now is to make one very quick attempt to disconnect, and if that does not work immediately fin down whilst isolating the cylinder and dumping gas.
I am a big fan of how The Human Diver - Counter-errorism in Diving - Home Page discusses this sort of decision making in an intelligent and non-judgemental way. Very interesting to learn about why we as humans make poor decisions.
I can not stress enough how some warm water only instructors underestimate how difficult some stuff is in cold water, like disconnecting a qd hose and the studet getting training in warm water but doing diving in cold will revert to training, when it may not be the most likely to work route. Your second route will work, the first ..sometimes, so why not do what you know will work first?