Both. Assuming you're not level 50 and going after a relic already, you'll need to make a choice as to how you want to play. Tower shields offer higher block strength with lower block rate. Bucklers are the flip side off that with higher block rate, but lower block strength. While leveling I carried two different shields with me until I was able to use Kite shields (which offer a balanced mix of block strength and rate) and finally Relic.

What to use varies on your situation and personal preference though.