Just to add what Grant says: Both Express and Developer Edition are free. Express has lesser features and a cap on the database size, but it is licensed for production use. Developer has all the features, but is not licensed for production.
You need to have an idea of what you will use in production. If that is Express Edition - develop also on Express Edition. If you aim at Enterprise Edition - no problem with Developer Edition. If you aim at running Standard Edition in production, developing on Developer Edition is likely to be the best choice, but you could accidently make yourself dependent on something which is only in Enterprise Edition.
Both Standard and Enterprise Editions are paid editions - with quite a bit of difference in the price tag.