With a pellet grill you can cook foods at lower temperatures for smoking or higher temperatures for grilling searing. Whereas a griddle is mainly focused on high-temperature searing, where smoke production is not on the table.

However, there are now pellet-based griddles (Recteq SmokeStone) that can provide a bit of smoke flavor. Well, more than a gas/propane-based griddle can in any case.

You also don't have to choose between a pellet grill or a gas griddle, there are several products that combine the two cooking methods, such as the Camp Chef Woodwind with a Gas Griddle or the Pit Boss Charleston as two examples.