Varnish was built for caching web apps. Squid is a forward proxy that can be configured to work as a web app caching program. So, when Varnish was designed we where able to disregard a lot of stuff that isn't needed when caching in reverse mode. On the other hand, squid has been around for ages and is a very, very mature product with a very well known set of strengths and weaknesses. Varnish is only 5 years old.
Varnish was built for caching web apps. Squid is a forward proxy that can be configured to work as a web app caching program. So, when Varnish was designed we where able to disregard a lot of stuff that isn't needed when caching in reverse mode. On the other hand, squid has been around for ages and is a very, very mature product with a very well known set of strengths and weaknesses. Varnish is only 5 years old.