I find it difficult to believe that most TMNT fans, including the ones who were born after 1991, prefer the syndicated series overall. We're mostly adults here at Toon Zone, but I kind of doubt that younger fans are on forums proudly declaring that the 1987 syndicated TMNT was THE definitive Ninja Turtles series. If you're truly going to make that claim, I'd like to see some official proof.
Again, none of us knows what's going to be remembered fondly 10 years from now, because we don't all have time machines parked in our garages. Suppose that some studio produces a new TMNT series within that time, and said series becomes popular and is a commercial and critical success. Are you honestly suggesting that even under those circumstances, that the syndicated series would still be the one that fans remember as being the best one? Heck, there are adults here now who grew up with the syndicated series who still prefer the 2K3 series.
Please, please, please don't start that "Batman: TAS>>>any and every other version of Batman" debate again, please. I'm begging you.
Anyways, regarding the voices, I'd say that 1987 voices work best for that more cartoony series, while the 4Kids voices work best for that series. I don't really have a preference.