in Dragon Age you can select race, gender, class, specialization, and it leads to a lot of combinations, in the witcher series, yo are a witcher, but geralt is a very complex character, and the world and other character you interact and meet are also very comple, there is almost no right and wrong, if you keep investigating, you will always encounter new and different choices,
you can develop different abilities of geralt, depending on your play style,
in dragon age is the fantasy a bit more colourful, i the witcher is like more mature, ther are controversial themes, like abortion, political corruption, racism (between humans and non humans) those themes are not in dragon age
in dragon age you ca be a mage , the witcher is a swordsman and he use some little magics as the "signs" only 5 spells,
the main weapns are two swords steel for humans, elves and dwarves and silver for monsters and magical creatures, he ca drink potions, to increase strenth, speed, see in darknes, etc
he can throw bombs, that's similar to dragon age
in dragon age there are more romance options, i the witcher there are more casual relationships, log term realtionships are limited to 1 or 2 as far as i know
in dragon age you control a group of 4 characters, doing all kind of missions all over the gae world, the help each other
in the witcher you are alone, for the witchers work alone,
in both games there are a lot of different dialog options, that lead to different outcomes
dragon age is all about the story, the plot, characters, choices, exploring, etc,
i totally recommend you to try dragon age inquisition, i'ts a wonderful game with a great story, characters and combat system 