I can't comment on joes, but I really like my cnc router parts machine. The r&p rocks either way u go I'd say do r&p. CRP's would be easy to expand to 4x8 just buy new 1545 x rails, cold rolled steel, cross supports and brackets. That said you could most likely do the same with the joes
When I was looking it appeared that the joes would be cheaper to build but more time to build it.