I assume you mean ATiTool in regards to "Ati Tools".
Anyways, these are really two completely different programs.
ATiTool is specifically used for overclocking the GPU and providing some other information.
ATi Tray Tools is a complete suite of settings for your GPU, it's pretty much a complete replacement of the Catalyst Control Center that is default with ATi Radeon drivers.
In regards to memory usage, neither use enough to be worried about, at current I only use ATi Tray Tools, I used to use them both since I liked the overclocking features of ATiTool better but I have stopped overclocking my GPU, seeing OCing my CPU having a much greater benefit. Right now ATi Tray Tools is using about 4MB or memory on my system, slightly high for just a tray app but considering that vast amount of features, I find that respectable and very negligible.