First of all, I would choose one in a general location of where they are more familiar with the people that are around, and that are close to their age...so that they will have things in common to talk about with others. The older generation does feel more comfortable in areas of familiarity.
I would also choose one that does not treat them like prisoners of their living community, one that makes them feel comfortable, with maybe something that is familiar with their lifestyle, something in common.
I would also check out the staff that checks in on them or that assists them with what they need assisted with. Me, even though they have laws now days to protect the elderly, I would make sure that they are really interested in your parents well being... I check in on my elderly friends to see that they aren't getting punched on or if they are comfortable with what's going on.
It's very scary when you have to start depending on others to help you and you not being in your own residence is scary enough.
Lastly, I would not put them there if I wouldn't stay there myself. Remember, you too are likely to need assistance some day.