The first question to ask is what do you want to do?
If you want to be a big gun, dx/contester on 10/15/20
Go with the C-31. Second choise would be the Skyhawk
If you want to have a good signal on 10/15/20 and still
play on the WARC bands the TH-11 is a great choice.
If so you might also want to look at the F-12 XR5.
The other two antennas you mention I am not too
familiar with so can't comment. But at VE6OA we
have a Skyhawk at 100 feet and have used TH-6s
any where from 35 feet to 65 feet and they work well.
The TH-6 isn't too far off of the TH-11 on 10/15/20.
Pictures from VE6AO
I hope this helps a bit.