I'm architecting an web application now. I think I could use varnish cluster as cache/load-balancer, and also I can buy a F5 hardware cluster(maybe three or so) equivalence.
How better about the F5 then the varnish/squid solution ?
Underneath the cover, the F5 boxes are regular x86 servers, with Intel-based hardware. I have never run benchmarks with F5 hardware, but I'd be surprised if their virtual licenses (which I did test), would perform differently, on the same hardware. The only exception is probably if you have lot of SSL traffic (and you'll get an F5 box with hardware-based SSL termination), then the F5's boxes will be faster for sure.
Both F5 and Varnish are a pain to configure, but F5 has easier/better configuration for clustering. For example, Varnish cannot share cache in a clustered setup or execute cluster-wide commands. If you can afford it go for the F5, but for a clustered setup, I'd honestly recommend something like aiScaler or Netscaler. It also depends a lot on your application.