I've gone for the separate mill and lathe.  The mill has a 600mm bed and the lathe around 1 metre.  I have to say I'd use the mill twice as much as the lathe, but both are great.  I re-surface crankcases after welding, reseat clutch baskets, shave heads and make all sorts of things.  Generally, I'd cast a new brake lever out of aluminium and then finish it off in the mill.  I Recently cast a brake stay arm to convert the HL500 rear rod brake to a cable brake.  That required both lathe and mill.  I also made a solid nylon chain guard to get the chain away from the frame, swingarm and ohlins shock.
Once you have them you'll find more uses than you could imagine.