G'day Cheeky, the VV has a stainless steel boiler, not brass, which should be more durable than an aluminium one (depending on what water is put into it). The difference between a stepped and stepless grinder is how the grinder is adjusted. On your Nemox, and other stepped grinders, there will reference marks on the adjustment ring, usually numbers. These marks correspond to adjustment 'steps' in the grind, so you might use the grinder on setting 3/4/5 for espresso for example. A stepless grinder uses a 'worm' drive to give you constantly variable grind settings. Depending on how fine the adjustment 'worm' is, you can tweak the grind settings by fractions depeding on how many turns of the adjustment screw. On most decent stepped grinders, like the Nemox, Rocky, etc. the difference between the steps isn't big enough to worry about and are fine for home use. A stepped grinder also gives you the advantage of easily moving between different grind requirements (if you have them) like plunger, espresso, turkish, etc. The above comments weren't meant to engage a stepped v stepless debate, only to inform of the differences between the two 😉. I know that Fresh Coffee sells the Challenge grinder, here's a link http://www.cosmorexcoffee.com.au/appliances.htm. In my un-expert opinion, I don't know what difference the portafilter size would make, ie 53 v 58 mm. If they hold the same volume of coffee, ie 7g single and 14g double, there shouldn't be a difference. Hope this helps.
