Litestep -- Just keep in mind that its dependent upon what you want to do with your computer. Depending on what modules you'll need to simulate the versatility of a taskbar, you may end up loading enough modules such that Litestep ends up using more memory.
If you're talking strict numbers, with no extra programs or modules for either side, Litestep definitely has a smaller memory print.
As long as you don't go crazy with modules and graphics, Litestep will almost always use less memory than Explorer. For a direct comparison, my current Litestep theme is using a shade over 7MB of RAM, whereas Explorer was using around 16MB last night.
the difference on win xp only matters if you got like 256 mb. Then you shouldnt run winxp in the first place, and your performace is shitty no math what. The mem difference between explorer and litestep might be 10 mb, which is exactly 1% of my ram. Nothing to care about ..
The problem I have with minimal themes is that the vast majority of them are crap and show no creativity at all. Why bother releasing something that anyone else could have done with the most basic of Litestep tutorial, and done in about an hour? There are some great minimals out there, but they're so far and few between.
To the original question, THC4k nails it, imo. Only on the most ancient of computers does it really matter either way. But, if forced to answer the question, Litestep itself really doesn't matter. It's the theme you're running and the features.
I use both Litestep and Explorer/WB5. Depending on my current attitude on the day. Recently I have been using Explorer more. Explorer only runs me about 4 or 5 MB's of RAM out of a GB. Litestep is about the same. So it doesn't really matter to me which I use. Just one has more customization then the other.
Advent: There is simple things you can turn on and off in Windows to make it drop from 18+MB's of RAM down to about 3 to 4 MB's of RAM... There used to be a website that had Major Windows tweaks all the way back to Win3.1, haha. It used to be: http://www.blackviper.com/ but that site has been under contruction for awhile now sadly. :/ Right now it is running at 6,280 KB's but that is because I have 18 Windows open.
While I was on Windows, I tweaked the hell out of my box. I stripped everything I could imagine down, and Explorer still guzzled no less than seven megabytes of RAM.