Generally Shimano's numbering sustem is lower numbers = lower line, higher numbers=higher line so the UN-55 will be lighter, smoother and more expensive than the UN-24. Square taper bottom brackets are considered "obsolete" these days so the choice is limited. The plastic cups are still used on some models and are a non-issue because they don't fail unless the installer does something wrong. All of the sghimano UN-XX bottom brackets are servicable, durable and good value.

I'm not sure about the Suguino bottom brackets. Their cranks, etc. are good but I don't think it's worth the huge price difference. If you really want the best square taper bottom bracket available at any cost, get a Phil Wood.