With a hanging rack, you have a lot more issue with the bikes knocking together leading to scratches and other damage to the bikes. Platform racks eliminate this risk almost entirely (and are significantly easier to load up vs. a hanging rack).

I haven't researched what 5-bike racks are available in depth, but a quick google search showed me that the only options for 5-bike racks were hanging racks---no platforms. If this is the case, I would opt for a 4-bike platform rack and a roof rack for bike number 5. But if you don't care so much about bikes knocking together on occasion when driving, and the longer load time, then a 5-bike rack should be fine.