Stradivarius is the most beautifully sounded violin manufactured by Antonio Stradivarius. It is also the most expensive violin in the world! There is no other violin that can be compared to this limited (only hundreds of it) Stradivarius sound. The credit goes to the type of wood used, some say it's the glue,the shape and lots more that we don't know. So there had been some attempt to reproducing the violin by Joseph Nagyvary, who is a biochemist. He managed to prove that he had found out the secret of Stradivarius violin when the violin he made (Nagyvary) only differs in small margin compared to Stradivarius's sound! Imagine how long he took to find it out! A Stradivarius costs several hundred millions to own..and the highest price ever paid for it was US$3,544,000...wow, look at how much people are willing to pay for a beautiful music and satisfaction. For a Stradivarius, it's worth the money.
A Guadagnini, on the other hand,is made by Giovanni Battista Guadagnini, which is reffered as "poor man's Strads"....meaning the more affordable alternative to Stradivarius in 1970's . But now it's not cheap anymore, it's expensive too.
Dr. Joseph Nagyvary and the violin he made (sounds very similar to a Stradivarius..such a genius)
And click here to compare a Stradivarius and Nagyvary sound.
References: Wikipedia and Yahoo! news.