For what it’s worth, there’s usually a baseline standard of deliverable that you’ll get with a professional design firm with some kind of track record. Their work might not be particularly great or imaginative unless you’re willing to spend top dollar for their best effort, but it probably won’t be embarrassingly bad and they'll usually meet their deadlines and promises.
A freelancer runs the risk of being flaky and producing crap, but will probably be significantly less expensive. Don’t forget that many agencies just outsource a lot of their extra work to freelancers anyway. You also have the chance of one knocking it out of the park and giving you something great for low cost.
Personally I’d spend some time surveying designer type sites and finding a few freelancers that have produced some work that emotionally resonates with you and engaging them in discussions and rolling the dice.