I would say Kerl because it is actively developed by Mark Allen from Basho (as I type this the last commit was six hours ago), and even though it might have an awkward interface and some concepts one has to grasp it is pretty good.
One thing to notice though, you need to build your Erlang versions with wx enabled if you want to be able to use :observer.start and other tools build with wx. To do this you can add the following to your ~/.kerlrc file:
KERL_CONFIGURE_OPTIONS="--without-javac --with-ssl=/usr/local --with-wx"
edit: please look at this comment for a .kerlrc configuration: Plug POST performance
I have only tested this on OS X though, ymmv.
