Congrats on your new P's. Yeah, with the exception of piraya, most pygo's colours fade considerably with maturity. I had a mixed pygo tank (2 piraya, 2 ternetzi and 4 cariba), but it was over a decade ago. I might be remembering wrong, but I think my ternetzi were double the price of my cariba and my piraya were a little more than 1.5x the price of my ternetzi. I know things have changed, but I'm kind of surprised that ternetzi are more expensive than piraya now. Personally, I don't think they're worth it at that price. Of the three species, the ternetzi were my least favourite (but still better than captive-bred natts)
All fish are different, but in my experience, cariba are the most aggressive feeders. They also develop the most bulldog-like head (which I love). Piraya, of course have the nicest colours and huge growth potential. While ternetzi are basically wild, yellow natts (I think they're from the same river system as Salminus brasiliensis). Since they cost more than piraya now, I would try to get either super reds (from northern brazil) or just regular wild natts instead (if they're available and affordable). Mind you, super reds will end up looking like regular wild natts when they mature anyways. Both will have a much heavier jaw than a captive-bred natt though.
Super Red
Regular wild natt
But who knows, if you decide to go with a ternetzi, you could get lucky and end up with one like this:
A specimen like this would be worth it IMO.