Hi Moldcore, IMHO, stick with the 64MB and then get the networking if you have the capability in your shop. This will get 99% of all shops what they need to run larger programs. You can always DNC lager programs from the network.
IMHO, The 256 meg option is too expensive, The "USB drive" is just an emulator of a 1.44MB floppy disk. If you have a 2 GB thumbdrive ( max allowed by DOS) you have to format it to be a 1.44MB drive. not very practiable in my opinion. instead of having a bunch of floppys laying around you have a bunch of thumb drives laying around.
See my other replies regarding DOS and hot swappable USB drives, yes it is possible.