Aria of Sorrow is definitely the harder of the two, but it's really good, the best out of the GBA games for sure.
Symphony of the night is similar to it, but just know that instead of stealing souls you just get sub weapons. However, you can input spells in the way you would a fighting game, which is really useful. Like this spell.
That's my biggest advice for the game, because I thought you had to unlock spells, but you don't, you can do them right of the bat, but they do require energy from your magic bar.(So basically, if some don't work right of the bat it means that your magic bar needs to be bigger, which it will be as you level up.)