BTO flats have income ceilings and other criteria (such as no private property owned within the past 30 months), while resale HDB flats have no income ceilings, and you can choose almost any flat type (except 3Gen). For BTO, if you are single and aged 35 or above, you can only apply for 2-room Flexi units in non-mature estates. For resale HDB, singles at least 35 years old can buy most flat types (excluding 3Gen). Both options may come with different CPF grants. Ultimately, it depends on your eligibility, preferences, and whether the income and property ownership rules for BTO or the flexibility of resale better fit your plans.