Posted Price Versus Competitive Auction

A seller can either (i) post a take-it-or-leave-it price to a single buyer with value $\mathrm{Uniform}(0,1)$, or (ii) run a no-reserve second-price auction between two buyers with i.i.d. values $\mathrm{Uniform}(0,1)$. Compute the seller's optimal expected revenue in each mechanism and compare them.