Currently I am using Squid3 on my netbook, which has an Atom N450 processor and 1 GB of RAM running ubuntu. The proxy is used to share the internet connection with a small group of PC's (3 machines) over ad-hoc wi-fi. Another reason for using squid cache is to bring down data usage, as my internet connection come with a usage limit of 2 Gbits per month. I found that squid3 is little heavier for my netbook, as the processor fan will be always active whenever squid is being used. I have plans to go for Polipo, as it is lighweight, and better performance results according to this test. But in the documentation for Polipo, there is no information about HTTPS support, and I always access sites which use HTTPS on my LAN Pc's. So, would Polipo be a better solution for my requirements? Efficient data caching is my first priority, and cached data shouldn't get deleted during reboots.
Performance differences between both for such a small setup are probably not noticeable.
However, traffic over HTTPS is NOT cached by squid3 anyway, as far as I know. So if you use the proxy to save bandwidth, all HTTPS traffic will not be cached, so you are not saving anything.
In any case, Polipo is more lightweight, so give it a go. It's also much easier to configure.
Squid is much more powerful, and more oriented to "enterprise" environments. I manage a Squid server for the school where I work, and Polipo just isn't an option.