In my experience the key is in achieving synergy between arm, cartridge and phono stage to best match the musical presentation you prefer. This is where I've found bake-offs to be invaluable in simply experiencing what bits of kit work well with each other.
I own an Aurorasound Vida and have hawked it round a few bake-offs and although it's a fantastic phono stage (IMO of course) it hasn't always gelled with every cartridge.
As Lurch has said you have to get the mechanics right first because a badly set up arm and cartridge will sound poor regardless of phono stage.
Then as bigfool1956 states loading becomes a big determining factor. For this reason I can see the attraction of phono stages with variable loading particularly if you plan on using multiple arm and cartridge combinations.