Well it's really just about what you're looking for in a game. If you want a good story, a fun cast of characters, the ability to do whatever you want when you want, customize your own unique character, and just have a good time, then go with Saints Row. If you want good fighting techniques, a solid story, a great arsenal of weapons, and thrilling sandbox levels, then go with Assassin's Creed.
Personaly, I've played every single Assassin's Creed game on console and loved them all. For Saints Row I didn't play the first, but I played the second and it was phenominal! It's not easy to decide on one of these great games, but if I had to choose, I would go with Saints Row The Third because you can do way more in the Saints Row universe than in the Assassin's Creed universe. The choice is yours.