Jim, yes I saw the price, I converted the £175 to $ to make the comparison easier Doing it the other way its the same, $195 for Ethernet SS from the USA is £121 so all depends on whether the UK one includes VAT or not. It would probably work out about the same (USA likely slightly cheaper) if the UK one includes VAT and the one from the USA was caught by customs. If the UK one doesnt include VAT and the USA slipped through customs then USA is a lot cheaper.
Regarding them being made in China, I am not sure, last I knew Greg got the boards made up and populated them in the USA but maybe that has changed.

I would say go with Ethernet, its more expensive but should be more noise immune. I never had problems with the USB one but a lot of people did.

The  all in one, is it worth it? Well for me no but for others maybe. You are getting a USB SS, a computer and a monitor with Mach installed so I assume also the operating system. Whether Mach is licenced or you have to pay extra I dont know. So suppose it may be a reasonable deal if you dont want to buy the computer/screen/operating system etc separately but you will be paying a bit extra for not doing so.
As for it being good for a dirty workshop then its no different from any other computer really as it is just an off the shelf eebox.
Hood

The SmoothSteppers are designed and built in the USA 

I ship them all over the world. Shiiping by USPS will cost about $18 and take about two weeks for delivery.