I would say Virginia class is superior to Astute because of three reasons
1- Astute class is based on Virginia class simply because without technical assistance provided by Electric Boat, the BAE Systems would never have completed the project
http://news.bbc.co.uk/2/hi/uk_news/england/2930237.stm
2- Virginia class is a reduced version of USS SEAWOLF which is the most sophisticated nuclear sub ever built. One SEAWOLF (9600 tons ) cost more than a Virginia class and Astute class combined !
3-The Electric Boat has invented the nuclear sub in 1954 when they launced the USS NAUTILUS.. Britain learnt from the US how to build a nuclear sub.
The first British nuclear sub, the HMS DREADNOUGHT had a Westinghouse nuclear reactor , all other British nuclear reactors made by Rolls Royce were based on US technology.
Astute class being the most complicated sub is nothing more than a BAE SYSTEMS marketing mambo jambo in order to make more palatable an extremely expensive and delayed programm to British tax payers, this is what they want to hear.