The C-5 actually has advantages:
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for some reason, power metering devices don’t work on a C-7.
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C-5 doesn’t have the struggle pairing Z-Wave that C-7 has
Advantages of C-7:
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can do S2 security pairing (C-5 can only do S0), which uses 1/3 the packets as compared to S0;
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supports the Device Firmware App for OTA Z-Wave firmware updates (C-5 has to use the no longer supported community driver);
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C-7 can run Tony Fleisher’s (@tony.fleisher’s) excellent Z-Wave Mesh Details app, C-5 can’t.
I have one C-5 and two C-7 (one is a backup for my production C-7).