[MENTION=25939]japespszx[/MENTION]
It really does not matter. Its down to personal preference. Chronologically, F/SN was written first by Type-Moon (TM), thenafter they collaborated with Nitro+, Urobochi Gen then penned the F/Z series. F/Z events took place before F/Z.
F/SN has certain short flashbacks the current F/Z details and history, but thats just that. It does not make any too much of a difference.
My suggestion:
Go for F/SN first.
Why?:
F/Z has overwhelmed the original F/SN by far. F/SN wasnt a slouch, but F/Z is too good to compare with currently. The older F/SN was animated way back by a 'poorer' company in DEEN, but I wouldnt say it was a bad job. Like WsE said, watching F/SN first does not ruin anything in F/Z. I would say you will regret more if you went for F/Z first.