I think strays are more appreciative. I mean, put yourself in their shoes for a minute... You're outside, alone, in the cold, hot, rain, snow, hail, etc, eating garbage outside of bars and restaurants that makes you sick but you're so hungry you eat it anyway, you have fleas and worms, you get in fights with other cats, etc. And then someone brings you inside, gets rid of your fleas and worms and ear mites, gives you a big comfy blanket to sleep on, and a bowl full of food that actually tastes good. How could they not be?
At first Zissou was so sick she basically laid around and did nothing, cause she had fleas so bad she was anemic.
That's not to say they don't quickly turn into spoiled brats, of course, but they always remember when they finally got a home. And other cats can be just as loving and adorable, there's just something about former strays, that makes it worth all the extra effort it takes at first.