Quad advantages:

1. Simple. Only wires and coax. No baluns, traps, gammas, hairpins,
etc. Full legal power continuously with no problems.

A yagi can be made with a direct dipole feed, which is very simple. BOTH
the quad and yagi need a balun.

2. Broadband. Covers 20/15/10 with less than 2:1 SWR, even on 10.

See Cebik's studies of quads (www.cebik.com). Quads give a very
narrow front/back bandwidth. Not enough even to cover most amateur
bands with = 20 dB F/B.

3. Light. For a given level of performance, a quad should be a bit
lighter than a yagi.

True for a 2 element quad (boomless), but not with more elements. Look
at some of the weights at http://www.mgs4u.com/catalog/.

Torsten
N4OGW