[syslinux] Memory usage with memdisk vs initrd a squashfs

[Ian Bannerman]

Question, 

Some Linux distributions I support show two approaches for pxe booting them. One uses memdisk to boot an ISO file. The other uses pxelinux to boot kernel and initrd a squashfs file.
I'm unfamiliar with the impact to memory either approach has. Like, does using memdisk permanently squirrel away a chunk of memory for the ISO file that the OS will be unable to use? Is using initrd to pull down the squashfs file more or less efficient? 
I would appreciate any insight! Apologies for the noob question :)
--Ian