Well, my friend Ozzy62 went thru this whole exercise just a few months back! I hope that he sees this thread & chimes in. Basically, he tried the KOntrapunkt-B (the more expensive one), Shelter 501 & the Benz L2. I think that he might have tried a few other MC cartridges but I cannot be sure. ANyway, he selected the Benz L2 over the lot ($800 new). His personal tastes aside, I believe that the medium compliance Benz L2 was a much better fit for the unipivot Spacearm. The Shelter 501 is a low compliance cartridge & it is likely to not match up the Spacearm. There are tonearms that can take low compliance cartridges but I do not think that the Spacearm is on that list!
Hope that this helps.
Hope that this helps.