The memory consumption is really up to what you do on the server. If you have an idle plesk server and an idle cpanel server, they will probably be pretty much the same. Cpanel has become much much more efficient as of cpanel11, so I would say cpanel is better.
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It depends on the implementation really. Head to head on a dedicated server, I'd say they're probably about the same resource-wise, assuming you have the same features enabled. However, cPanel has a lot of extras that you can install which can consume more memory if you enable them.
Now, on a Virtuozzo based VPS (and only virtuozzo), if a provider is using the cached plesk application templates, plesk will kill cPanel *EVERY TIME* from a resource standpoint. It's really a slick setup in that regards.
However, for the most part, I really am not a fan of plesk other than that one instance i just mentioned.
When it comes to VPS the scenario is again different . It is better you go for still less memory and space consuming control panels like Direcadmin , though features in Graphical interface are much lesser compared to Cpanel and Plesk Still it has most of needed features for managing a domain .
I am not sure whrss, as the provider are giving cPanel free of cost, there was an option of cPanel or Plesk, so I suggested my friend to go ahead with cPanel based on the suggestions
alanz - I prefer directadmin too, but there was no directadmin free discount by the webhost, so cPanel was chosed ahead of Plesk