Wire Flex or Aluminum Flex for Heater Duct

I need to run about 15 feet of ducting to a floor register.
Should I use Wire Flex or Aluminum Flex?
What are the pros and cons of each?
The diameter is 8"
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If there is any way possible, I would run solid metal.

--
Joseph Meehan

Dia \'s Muire duit
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