I personally would have slower music. Generally it is only background music while your guests have their dinner, and not too many people will be listening to it as such. However if the music is too fast, they will subconsciously feel rushed while they're eating. On the other hand, if it's slower they will feel relaxed while they're eating (but I'm sure they won't fall asleep).
I think anything Michael Buble is a good choice, I also like Brooke Fraser. If you want you could go some of the older stuff, like Fleetwood Mac, Elvis, etc etc.
I agree! Slow easy listening type should not disrupt the guest. And also, to make sure the music to be played at a moderate volume (preferably backgroud music volume) is, I think it's important. If you imagine a fancy restaurant and how they usually have all these easy to listen tracks in a softer volume that might give you a better idea of what I mean! Likek mini_l27 said, fast music should be avoided too.
If you like, I think something like slow jazz music, brazillian (like bossa nova), ballads are the safe choices to make.