After some further banter on Twitter about why fixed-bid projects are better than hourly. I decided to put my points down here. The post says_

If you respond to the question with your hourly rate, they may start immediately comparing you to other creative professionals, despite stark qualitative differences, essentially comparing apples to oranges. Or they may stop listening to the carefully constructed explanation of your creative process and start calculating what they think it should cost or, worse, how much you earn, perhaps compared to what they earn. Don’t let them go there.

I've highlighted the sentence I want you to pay attention to. You should never worry that clients are comparing you to others. Not if your skills speak for yourself and you have sold yourself correctly. The bigger the client, the more likely they will be comparing you against others. Being insecure at this point is a sure-fire way to lose the bid.

Fixed-bids don't often end well in my opinion. Ideas are ever-evolving and this method of work doesn't consider that. Scope creep is your biggest threat here and there are 3 outcomes to this_ There is more work than originally expected - you don't get paid for the full hours you put in, there is the same work as you originally expected - everyone is happy, there is less work than you expected - your client gets less work than he paid for. Scopes change, so chances are someone will be unhappy here.

You are not a sandwich shop! Your materials are low, and your hours are high. Therefore as a service job, we bill hourly. I understand people's issue with billing in their less productive hours but if you bill for your more-productive hours I do not think that's fair. It's not as clear-cut as fixed-bid versus fluid per-hours. Billing hourly doesn't have to be unorganised. Work out max-budget or max-hours, keep the client in the loop and the client will be happy. We deserve to be paid for all the hours we put into a project, why accept less?