I stayed at Hacienda San Angel and if they are recommending the trio that plays there for their cocktain hour, it is totally worth it. They were wonderful- can do popular songs that all us American girls know, local love songs, more upbeat local tunes too. If your guest list is 20, then I would recommend skipping the mariachi period. Mariachi bands are usually 10-12 people, and so that might be a bit overwhelming considering that equals have your guest list. You can just have the trio start mellow and then go more lively, as that is what mariachi is anyway. Also, I'd stick with an iPod too because hiring a DJ for 20 might be much too- consider how many people will actually dance and do the math from there. Hope this helps.