Block Rate vs Block Strength

[WingsofWar]

Generally to be an effective Tank for most play styles: Kite Shield > Tower Shield > Round Shield

Kite gives you best of both worlds a pretty even mix of strength and rate.
If you cant get an equal kite shield for your level, tower shield is a better choice IF you can bump up your dex and str & parry stats using materia. + when you unlock Bulwark which grants an increase of 60% block chance it becomes a damage mitigating heaven for those few seconds.
Sure Round Shields give you good rate but a shitty mitigation stat, which diminishes your ability as a tank effective roll. HOWEVER I did found Round shields really effective if i'm try to bump up my DPS more, or slightly generate more enmity than an equal level tower shield, as round shields usually have better acc/determn/skill speed stats.

If you actually had a meaningful choice in shields this would probably matter.

^ And pretty much this.