S14 Has SR (nuff said) stronger gearbox internals, better viscous diff (kouki S13 has viscous but it was barely worth having over the open diff in the zenki)
Like mitto says the S14 is easier to learn in, as everything happens a bit slower and they feel less twitchy especially compared to stripped out S13
S13 shell feels lighter and more nimble
S13 has less electronic bolox to strip out (abs airbags etc)
Also S13 has been around longer and has the Silvia variations and was marketed in many more guises and therefore there are more off the shelf parts available, but the S14 is catching up, it is even easy to get a nismo power brace for one now so I think its just a matter of personal opinion.
If you are building a drift car its what you put into it in terms of time and money and individual taste that makes it what it is i guess...