If you plan not to get any discs at all then Super Slim, as you can change the hard disk easily and store games on the hard disk/SSD.
The Slim is good on an overall basis but getting a very good condition console is tough, it is the most pragmatic and in my opinion the best looking.
Don't get the fat now as its a big pain for many things in this day and age (the CR2032 battery is a chore to change, many fats have their RSX CPU overheat and only delidding helps along with YLOD - Yellow Light of Death issues)
Also an another point I'd like to make is although it is very controversial I would say install a 1TB SSD too, you can do it on your own so no worries there.