Serenity: The Roleplaying Game
The system itself is pretty simple and is designed to flow smooth. In some cases I found myself wishing for a little more detail, but then again when have I ran a system "as is"? And the simplicity is good for more casual gamers who don't want to use calculators or five different books to make and run characters.
The basic mechanic of the system is that your stats and skills are both expressed as dice, with the higher the die (as in d12 versus d4), the better. There are 6 stats, Strength, Agility, Vitality, Intelligence, Alertness and Willpower. Skills are in groups such as Guns, Unarmed Combat, and Piloting. You can go up to d6 in a Skill Group. Beyond that you need to specialize, so to you could have d6 Guns with d8 in Pistols.
To do something, you roll the appropriate stat die and skill die and try to beat a target difficulty. Certain things may make your die step up or down and opposed actions such as combat are usually against the target's appropriate skill and stat dice. So if my character with d6 Agility and d8 Pistols was shooting at a Goon with d6 Agility and d4 Dodge (he is just a Goon), I would roll my dice and the GM would roll for the Goon.
The odds in the above example would be in my favor, but also illustrates the big difference between S:RPG and d20. Mid to high level PCs in d20 can mow through low level goons almost with impunity. A low level goon might have +4 Attack Bonus in d20 and probably about the same Defense Bonus. Meanwhile a mid-level PC could have +10 AB and around +8 DB, which mean the Goons need 14+ to hit while the PCs need 4+ to hit (on a d20). Then figure the Goons will have around 20 hit points while the PCs will probably have around 50.
In the S:RPG example the PC's average would be 8 while the Goons would be 6... I don't know what the percentages are off the top off my head, but as you can see, there is less of a spread. And since characters in S:RPG don't gain "Life Points" easily and even the toughest PC doesn't have that many, even Big Damned Heroes need to be careful.
Which one works better depends on the feel that you are going for. D20 is more cinematic, S:RPG is grittier.
Overall, I'd say 4.5 Browncoat Flying Monkeys for Serenity the RPG.