There are pros and cons to both.
As smartphones can do so much these days (blocking numbers, hidden folders, password protection) it is possible to run a pretty undercover ship with one. I use mine for communicating with civvy girls I'm interested in. You need to know the phone and its technology back to front though, as there are many ways of slipping up that may never even cross your mind. For example, iTunes actually backs up your text messages every time you sync and they can be accessed through your computer.
The main thing that stops me using my real number with prossies is that I think it compromises my anonymity. It is possible to find out more details about a person if you have their real mobile number.
An unlocked smartphone and a second sim is a good all round option in my view.