Churchill because Hitler decided to invade Poland, an ally.

Roosevelt because Hitler decided to declare war on the US in acknowledgment with Japan.

In regards to Churchill, he actually had no power when this happened. Chamberlain was still prime minister and he decided to go to war because it was clear that the policy of appeasement was no longer necessary. War had broken out and there was no more time to be gained from continually pushing back an inevitable war.

Roosevelt also had no power over the state of war (though he wanted to join with Britain) because it was not him who declared war.

Now, in regards to an alliance, there would have been no chance of that ever happening between Hitler and Roosevelt or Churchill/Chamberlain, not in a million years. The three powers were so ideologically separated that the result would have been simply bizarre.

First, there was no threat to justify the alliance besides the USSR, but the USSR was working against the enemy of Britain which removed that threat entirely. Second, an alliance would have only made sense to buy the US and Britain time to develop a war economy and combat forces to combat the Axis and/or the USSR, which was again unnecessary because the state of war was already opened and the threats were dealt with.

The question is largely flawed. It was not the choice of either men or countries whether or not war or alliances with the Nazis or the Communists was better as this choice was already made for them by the Nazis.