(Part of) the Arc community is discussing NewLISP: http://arclanguage.org/item?id=2887
I don't know enough about NewLISP internals, so I am the wrong person to take part in this argument...
It's a fairly pointless discussion. Arc and newLISP have different aims and uses, so comparing what one has that the other hasn't - and vice versa - isn't too useful. newLISP is a Lisp-like scripting language; Arc is a macro-package built over Scheme (I think - I've never used it).
Use whichever one 'floats your boat'. What's cool is that neither are Common Lisp, and people are enjoying using the language of their choice.