I have tried both, started with FNB in Feb this year when I started working. The "Premier Banker" didnt even know her own products, opened up the wrong account for me and about a month later was the correct account only opened. Never had a good experience dealing with FNB "Premier bankers". Their email and hotline responses are no better than any normal banking channels.
Approached Standard Bank in November, within a day I had a private banker phone and setup an appointment to go over the details of the account. She had the account opened within 2 days and cards ready very soon after. Service i've received from Standard Bank has been worlds apart from FNB's. Any requests are dealt with within a few hours as opposed to a few days by FNB. Even the switching over hasnt been a hassle as the banker sorted out my debit order and salary switching.
Im sure there are others out there who would disagree, but this was my experience with the banks. Would definitely recommend the Standard Bank option.