It's hard to characterize his main argument, but it seems to be that he prefers bombers because they can be recalled once they are sent off. This seems reasonable at first glance. After all, who doesn't want the ability to avert thermonuclear war? Who doesn't want the ability to say "never mind" after the bombers are on their way. It does seem strange that both subs and ICBMS can get everything done in less time than it takes to get across some towns. Wouldn't it be better if everyone had time to cool down and talk things out?
There's only one problem with this. Under what circumstances would we launch NUCLEAR WEAPONS where there was still room for discussion? If we are using nuclear weapons as a deterrent, then our use of nukes is simply to respond to an attack. To hit them as hard as they hit us (or harder), so that they and everyone else know not to hit us again. If we're actually willing to turn the planes around due to negotiations, that says that we are not using them as a deterrent after all, but instead as nuclear coercion. That we are engaging in a first strike.
He's almost explicit about this.
The nature of submarine warfare and the security of missile sites in the US mean that there will be no "warning" of attack to an adversary to make them negotiate. No, the first time they know the US is serious is when a mushroom cloud appears.
He ends it with:
Short of full nuclear disarmament, however, I'd prefer Slim Pickens flying in a recallable bomber than two guys in a silo in Nebraska with funny keys around their necks.
Recall that, in the end, Slim couldn't be recalled either. These are not weapons to be used for any but the most serious purposes, and you want to be damned sure you know what's going to happen when you do. Give me subservient computers and the guys with the keys around their necks.