Personally, I think it is best to take out airbases. 1: because of psychological factors (I'm talking about experienced players here, who would miss being able to restock with fighters), and 2: they are less likely to waste ship-bourne bombers on my cities if they might be needed to fight off someone else's navy which is edging ever closer to their shores
In all cases I am assuming that all my land-based assets will be taken out by silo launches, though I should be able to protect my silos from subs and bombers long enough for them to launch most if not all of their nukes.
Really, the question is this: is it wise to 'waste' 6 nukes to take out an airbase before you have taken out the silos around it?
(answers on a postcard....)





