I would suggest smart-casual, unless you are very comfortable in suits. Its stressful enough without worrying about being uncomfortable in a suit.
Where are you applying and to do what?
I went smart casual to one of my interviews, and for the interview at my undergaduate i went straight from the gym! I think each department is different in its norms. I got ESRC funding from both!
So the lesson her is its what you say, not what you look like that really matters, but still, don't go looking like a tramp, noone wants that.
BTW you could always drop your potential supervisor a quick email, they'll know what the interviewers are likely to respond to.
Best of luck.