Short answer:
Go for 40Mhz channelization (at least) in the 5Ghz spectrum.
You generally place APs close enough together to provide good 2.4Ghz coverage.
5Ghz spectrum does not propogate as well and there is more 5Ghz bandwidth available.
If you go for max bandwidth (channelization) on an AC capable AP, you've cut yourself down to 3 usable 5Ghz channels.
That's not any worse than the 2.4Ghz situation and (since 5Ghz signals don't propogate as far) you're still less likely to get interference in the 5Ghz band.
(assuming there are no 5Ghz cordless phones, etc.)
In general, I let the APs choose their channelization, and they go for the max that they're capable of.
I don't have many AC capable APs but so far I don't think this has caused me any problems.