Ah, the disadvantage of choice.

From the benchmarks I read WebKit uses a bit more memory than Gecko. Gecko was faster though (with Tracemoneky) but now both have a new JavaScript engine. You might like Tear it is also using WebKit, and runs native on Maemo not requiring Deblet. Midori probably uses a lot of memory because it runs on Deblet and requires its own libraries (e.g. GTK & Hildon); it cannot use the shared Maemo libraries which are already loaded.