So, when feeding a VFT, which is more important to consider- the fact that feeding a trap might cause it to blacken and die afterwards, so maybe choose an older one? Or the fact that a newer trap might be better at catching and digesting the prey?

Hmm. I would say any healthy fully mature trap would be possible.

Old traps will normally die during the digestion process, assuming they are on their way out already. For this reason, I usually look for newer traps that have recently opened and set for the first time to feed.

I usually look for a mid-aged trap. They usually are very ready to snap shut but have had time to develop their full color. Sometimes individual traps will stop developing redness after eating, so I'm told. I can't remember if I've ever actually observed this.