Different, but better.
Aria of Sorrow does the best it can to mix in the best of the old with the best of the new. Aria of Sorrow borrows TONS of elements from Symphony of the Night and anyone who enjoyed that one will love this.
Aria of Sorrow begins in the year 2035 with Soma Cruz and his childhood friend Mina in Japan. Suddenly an eclipse opens up and they find themselves in Dracula's castle. Surprisingly enough, it really looks no different than it did in say... 1797 (Symphony of the Night). It bears a strong resemblence and any Castlevania fan will notice all the different places (Libraries, Catacombs and the like).
Gameplay in Aria of Sorrow is very close to SOTN in several ways. Your character resembles Alucard in the way he looks and walks. Perhaps what separates this from the past two GBA releases is that your main character doesn't use the whip as his primary weapon. Actually, you don't really have a primary weapon. You can use, Swords, Daggers, Axes, Javelins, whip swords and more! Its fun to experiment with all the different weapons to see what fits you best. The strongest weapon may not always be the best.
Since, Symphony of the Night, Castlevania has gone into an RPG style character building stage. This is for the better and Aria of Sorrow is no different than the past games. As you kill enemies you'll eventually level up. It doesn't require too much to level up (at least not as much as it takes in Circle of the Moon).
Aria of Sorrow also has well balanced difficulty. It's harder than Harmony of Dissonance but easier than Circle of the Moon. Aria also throws in a unique twist. You can now absorb enemies souls at random. When you kill an enemy you may or may not get their ability. There are over 100 enemies meaning that the game even has more abilities to unearth than the DSS System allowed you to in Circle of the Moon! Most souls cost MP and that brings me to my first con. The game has a lot of souls but for whatever reason they cost A TON OF MP to use. Granted your MP regenerates but it regenerates REALLY slowly.
The game also presents lots of bonuses. You can unlock a sound test and a hidden character to play through the game as. There's also a boss rush mode and perhaps whats best is that you can link up with a friend and trade souls! Perhaps one bonus I would've really enjoyed would've been if they'd brought back the ability to play as different "Jobs" like in Circle of the Moon. But its still a really cool experience and the game has lots of replay value.
Visually, Aria of Sorrow is BEAUTIFUL! Your protagonist stands out in a crowd and the screen isn't so dark. One of the better looking GBA games that truly stands up to what the GBA is capable of.
I wish I could say the sound surpasses all else. Castlevania is known for having good soundtracks. The music is WAY better than the godawful soundtrack of Harmony of Dissonance, but it can't hold a candle up to the far superior soundtrack presented in Circle of the Moon. The sound effects are incredible and fun to listen too.
But Aria of Sorrow is not without fault. The most noteworthy con in Aria of Sorrow is that this game is short! It'll only take you 5-6 hours your first time through. The second is that while its harder than Harmony of Dissonance, its still rather easy for any Castlevania veteran.
Overall, Aria of Sorrow is an INCREDIBLE game. I still think "Circle of the Moon" is the best GBA release and its no contest that "Symphony of the Night" is the all time best but Aria of Sorrow comes AWFULLY close to being as good as those two.