a) How much do you think your skills should earn you in a year? b) What fraction of a work year will this project take? Multiply a and b.
Are you desperate for work? Wanting to give the customer a bargain? Wanting to rip them off? Worried someone else will underbid you? Learning on the customer's dime so it'll take longer than someone with more skill? Willing to haggle in order to try to float a high quote? etc, etc, etc. There's a zillion factors that are personal that you'll have to weigh for yourself.
Until you get more comfortable with this aspect, I'd say hourly. But you'd better to be able to give a good time estimate to the customer, and give the extra hours for free if you blow it majorly on how long it will take.