Not heard the Roksan, but have both a 401 and a 75. If I was to make a bit of a generalisation, idlers are generally more bass driven than belt drives. Slightly smaller sound stage, but make up for it with a musular, more "driven" sort of sound, whereas belt drives can sound a little airy fairy by comparison. Better sound staging though..
Kind of like the difference between SE and PP valve amps I guess.
Whichever you choose, unless you are buying a finished project which will cost a lot more, you'll be needing to do a bit of DIY. If you can though, both these decks offer pretty much unrivalled VFM.
“Music has always been a matter of energy to me, a question of fuel. Sentimental people call it inspiration, but what they really mean is fuel. I have always needed fuel. I am a serious consumer. On some nights I still believe that a car with the gas needle on empty can run about fifty more miles if you have the right music very loud on the radio”
Hunter S Thompson