(Note that rxvt and rxvt-unicode are different projects.)

I don't know how it compares to xterm but the manpages sound promising:

It is still faithfully following the original rxvt idea of being lean and nice on resources: for example, you can still configure rxvt-unicode without most of its features to get a lean binary. It also comes with a client/daemon pair that lets you open any number of terminal windows from within a single process, which makes startup time very fast and drastically reduces memory usage

In the faq:

Rxvt-unicode uses gobs of memory, how can I reduce that?
Rxvt-unicode tries to obey the rule of not charging you for something you don't use. One thing you should try is to configure out all settings that you don't need

With the right perl extensions loaded I can now select some perl code, right-click for a popup menu then "eval perl expression", the result is displayed and put in the clipboard. Well, I got excited anyway...