I have to say that I have always found flexicuffs softer and kinder to the skin, less likely to leave any telltale marks, and generally more agreeable in most aspects of general use.
However, it must always be remembered what the ultimate purpose for using any type of restraint is - comfort is not always the top consideration.
Indeed, degrees of pain are often the intended consequence of any significant attempts at movement, or in resisting the restraint, in which case it could reasonably be argued that the original style handcuff is to be preferred, and is therefore more suited to purpose.
I guess it all comes down to the overall benefit vs cost, particularly taking into account both the feelings and intentions of both the restraining party and the party to be restrained, bearing in mind that once in the cuffs (of whatever chosen style), the restrained party will have far less say in the matter.
Reflecting on your user name, 'Inbound On Descent', the question appears to be far deeper than I first imagined. A fuller answer might only be possible once we know whether your intended use was for when 'inbound', or receiving the 'inbound' traffic or whether the intention was only to use them when 'going down'.
I'd suggest trying both types in all scenarios and situations, and share your findings for the benefit of everyone.