I saw this google-groups post cross reddit this morning, and I really consider it one of the most lucid discussions on the differences between the two languages.
Others no doubt base their choice of programming languages on just such issues, and they generate the hottest debates -- but to me that's just an example of one of Parkinson's Laws in action (the amount on debate on an issue is inversely proportional to the issue's actual importance).
Myself? I've always found that Ruby's tendency to lean more towards perl-like syntax to be the turnoff (line noise for the loose), not the actual concepts behind the language, although Alex's points about "too much dynamicity" are quite good. Ruby does have a lot of good features that may eventually find their way into Python. Although, I'd much rather see Erlang-ish features without the dense syntax.