I'd consider a "desktop miner" as something that could run off a DC brick. Likely a USB cord, possibly its own ethernet connection, but if it requires enough power that an ATX or something is required it's past the cutoff for me.
Yeah, but then you get into discussion of what still qualifies as a DC brick. Does a laptop adapter count? You can get those well above 100W nowadays. I've even seen some 200W ones advertised for 'gaming laptops' - can't say I know if that's proper output or just throwing numbers around there, though
USB vs ethernet is a slightly simpler criteria, in that it probably shouldn't be one.. once things are externally powered, those are both relegated to just communication anyway.
If you are talking about a pure USB miner bitmain is probably your best bet.
They're solid little miners for sure. I'd go with the U1 myself - slightly less heat management needed and no ungainly heatsink on it - practically a copy/paste of the block erupter USB. Though given none of them will ROI and it's intended just for fun.. any choice will do, really.