Pillar 3a: tied pension
Pillar 3a is the "tied" pension: in exchange for capital locked until retirement (barring early-withdrawal cases), you get an immediate tax deduction up to an annual cap. It is the most tax-efficient tool to prepare for retirement in Switzerland.
Pillar 3b: free pension
Pillar 3b is the "free" pension: savings, life insurance or investments with no lock-in, accessible anytime. In exchange for that flexibility, it generally grants no federal tax deduction (some cantons allow a limited one). It's a useful complement once 3a is maxed out, or for variable-horizon goals.
Which to choose?
Simple rule: max out 3a each year first to capture the tax deduction, then consider 3b to save beyond the cap or keep flexibility (capital available before retirement). The two are complementary, not competing: 3a for tax efficiency, 3b for freedom.