IME, Ludwigia sp 'Red' will color up more in a greater variety of conditions. Ludwigia sp 'Rubin', although also a colorful plant, will show much a much greater variety in color depending on your tank conditions.
The following links are very informative (although not at all conclusive as, apparently, new ludwigia hybrids are brought to market):
The first link suggests that L. sp. 'Red' is not 'Rubin'. The second link shows a great comparison among several ludwigia species (although sadly not w/ L. sp. 'Rubin')...