Definitely a suit, it shows you're taking this as a serious business opportunity and not a trip to a casino. If you're going to be backed, they are looking for a return on their money and want at least what looks like a serious applicant.
You might get away with no tie but i've seen managers dress up to interview applicants and you certainly don't want to be under dressing them. You can always lose the tie and take the jacket off if you feel you've over done it but it's very difficult to go the other way.
At the biggest prop houses in London the trainess all start in suits and as you get more experienced you get away with dressing down more and more, in effect you earn it. When you're paying the company out of your trading you can wear what you like but whilst they're still supporting/paying you you'd better not look like you just rolled out of bed, or you'll be first to roll out the door!
Best of Luck