I've tried several of the cartridges in the under $500 range. This won wins, hands down, for me at least:
http://www.goldring.co.uk/record-cartridges-and-styli/1006.htm
Excellent sound stage for mono 45rpm playback as well as stereo. Crisp highs that are not shrill or sibilant; warm midrange that isn't boomy like I hear from the Shure M-44 or whatever it is called. and the bottom end rides smooth.
My pal and I had a listening session, as he purchased several cartridges, including a mono cartridge, for his Technics 1200. Since i also spin on a Technics 1200, I figured what I heard at his place (Marantz '70s receiver, good speakers, top notch headphones) would sound nearly the same on my set-up.
The Goldring G-1006 beat the mono cartridge, as well as the other 4 or 5 we auditioned. The mono cartridge ($750, can't recall the brand) sounded okay, but the highs sounded muted and the midrange was far too boomy. We had to eq out the midrange to get the sound balanced.
The Goldring required no eq via a parametric equalizer. I really judge my sound stage by playback through headphones, and again the Goldring passed with flying colors for mono and stereo playback.
I've used the G-1006 and the G-1012X, the 1012X is more $$$ but I really don't hear that much of a difference for the kinds of records I play.
I'm sure some of the high end audio folks will chime in with their preferences, but unless you have a $5000 or higher turntable, spending big bucks on a cartridge is a waste of money. $350-500 is the sweet spot.
i also would not bother spending money for a mono cartridge. You're better off wiring your pins from the cartridge to play mono, if that's what you desire. Since I use an Onkyo TX8500 receiver (1978 vintage) it has a mono / stereo button - good enough for me!