These two just jump up and down day to day, here is a recent sale of PSA7 and the 58 went for almost twice as much, but in the 5 and 6 range they are pretty similar with the 59 edging out the 58 on many, so much for my theory.
1958 PSA7 for $1052.00
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT
1959 PSA7 for $600.00
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT