What is wrong with your analysis is that a list of pros and cons, besides stating the obvious, does not really enable you to make a decision. Let's take a different point of view: you are a decider, you're either the President of the United States or leader or dictator of either Russia or China.
With such an information in your hands, you know there is gonna be a leak sooner or later, and that once it is public, you will have almost completely lost any control on the outcome - a news such as this will likely inspire either global chaos and almost certainly one or more military coups in either one of the countries which have space exploration capabilities and maybe could serve as ambassadors to the aliens.
So, being open means you'll stop being a decider. However, if you're a decider, its almost certainly because you have a high self-esteem, and you certainly think you are a better decider than others, who might become deciders at your place.
Now, this puts you under pressure: since the info is gonna leak, and once it leaks the outcome will be very negative - loss of control, you know that you have to be not only very secretive about it, but hurry in the making of any decision towards first contact before it leaks.
Remember, that after all, what is at stakes is not whether you or your country have a nice legacy, what is at stakes is whether humanity performs and then survives first contact.
So, provided you're not incompetent, secrecy is the only way to the go.
Now, you will be right and comforted in your decision, if you're the ruler of a dictatorship: by definition, no one there is more confident nor competent than you - for the good of humanity, you must act, and quickly. 385
Does such a reasoning apply to democracies ?
While the leader of a democracy can trust that there are almost certainly people better prepared to take the decisions to first contact, again, the pressure that those people will loose any capability to perform should seriously worry you. Therefore, all your decisions must be oriented towards enabling those people, astronauts and the most humane scientists, to perform first contact before the leak.
And that means, to stay in charge and be secretive about your objectives, even while threatening your democracy, à la 2012. As in that movie, I would certainly tell the news to the leaders of those nations that have the means to perform first contact, and be secretive about it - again, whether first contact happens or not under my watch is irrelevant, the only thing that matters is that the best possible outcome happens under the best possible choices.
And under that point of view, being open is almost null in terms of benefits, while acting quickly under secrecy remains the unique way of anticipating any sort of any possible future. 385