OK. Halls, you know what to expect - but, you won't necessarily have any control over who you live with. They could be lovely, they could be ghastly, and you won't know 'til you get there. It would be cheap and convenient, and there'd be no hidden costs (like bills).
House share - you get to live with people you know and like. Having said that, you should be careful that they're people who'll be responsible tenants (i.e. will they pay their share of the rent and bills and not trash the place) and who live a similar sort of lifestyle to you (i.e. if you're trying to sleep while they're trying to party, or vice versa, then you could all get quite fed up with one another). Cost could be very variable - houses vary a lot in price. Don't forget abut bills. You've got the chance to find somewhere a lot more home-like than halls - but you do have to find the place.
Studio - I'm surprised that you're finding cheap studios, because I would generally consider this an expensive option. Still, if the price is right, then the question is whether you'll miss having other people around, and really only you can answer that. How and where do you spend your time now? How would this change in a studio?