Ready At Dawn's PSP masterpieces translate wonderfully to the big screen.
First off, both games play like any God of War you have played before. If you haven't, you really should get the collection of the first two before you delve into these. Simply put, God of War is a tactical beat'em up game with context input moments, platforming, and puzzle solving. God of War took a large part of its cues from Devil May Cry whereas this series is more melee oriented.
Chains of Olympus takes place right before the first game, ending up right at the cliffs from the beginning of the first game. I found CoO to be a little closer to the first God of War. You're more focused on dungeon crawling and puzzle solving here. The value for the game is a bit limited. You have the Challenge of Hades after beating the game and a few unlockables.
Ghost of Sparta is more linear and takes place shortly after the God of War game. There's not as much back and forth as CoO and far less puzzle solving. However, GoS has a much greater level of value. After completing the game, you'll have access to two different sets of Challenges, the Temple of Zeus where you can unlock many different things, and a combat arena where you can practice your skills.
Both games have been converted to 720p and look excellent even when compared to PS3 graphics. More importantly, the games play in 3D. If you have a 3D TV, I highly recommend playing it in 3D. These games have some of the best 3D I have seen, even compared against PS3 games and 3D movies. Only the pre-rendered movies aren't in 3D, but even then, they are impressive considering this was a PSP game.
The audio is the same epic music the series has had since the beginning. Vocals are excellent. The dialog never feels wooden or cheesy, always coming off as genuine.
I would say that Ghost of Sparta is probably the better in gameplay of the two, but Chains of Olympus has the better story. Neither of these games is absolutely necessary to understand the series, but they do give much deeper insight into Kratos, humanizing him even. The only real drawbacks I seen were the enemy variety. You're still fighting the same soldiers/medusa/cyclops/etc. enemies you have since the first game. However, both games come with trophy support. If you're willing to spend the money, I say pick the games up.
The Good: Excellent visuals and sound production, 3D effect is stunning, trophy support, Ghost of Sparta has excellent value, both games help bridge the gap between games and fill in back story on Kratos.
The Bad: Enemies are the same since the original, Chains of Olympus lacks in value and is short.