The number of warheads each nation maintains is agreed on in the START treaties, and those levels are determined by stockpile effectiveness. The US is recognized to have superior targeting and guidance systems, so they need fewer warheads to maintain parity with Russia's stockpile.
The best possible outcome is for SDI and it's descendants to be a complete waste of taxpayer money. If some clever chap comes up with a practical missile defense system, Russia would immediately generate enough warheads to overwhelm such a system and maintain parity.
Each missile represents a potential fault path to WWIII. We've been lucky with at least a couple near misses in our history. I don't look forward to a future with more.
Have you seen Dmge ? It's web based, but it has fog of war and a few other nice tricks, if you've got the maps.