Why Householder reflection is better than Givens rotation in dense linear algebra?

Implementing the QR factorization with Householder rotations is cheaper ($2n^2m$ vs $3n^2m$ for a $m\times n$ matrix), and equally accurate in practice. See Section 19.6 of Higham's Accuracy and Stability of Numerical Algorithms, or Golub-Van Loan for more explicit algorithms.

Moreover, in a Householder-based implementation there is a higher fraction of level-2 BLAS operations vs level 1, which makes them easier to optimize on a real-world computer.