Shield width is the most important stat, as that affects your ability to block oblique attacks/arrows. The Elite Cavalry Shield is popular because it has good width (32) and speed, enough armor to stop most bolts, and it's fast and has a nice texture. The Heavy Round Shield adds more durability and coverage, at the expense of some speed, but it's also very popular. The Kite/Heater shields require you to face more directly at your enemy to block their attacks, so that's why round shields are more popular.
The shield speed stat mostly affects your ability to feint (block in the middle of an attack), which is important for 1h, but probably less so for hoplites.