If by "good" you mean blue ribbon or first across the line, I'm not
sure there is such a beast who could do both -- you're talking about
specialists of high order in very different disciplines. But if you're
talking personal best, get a horse, any horse, whatever you can live
with. I'll put a plug in for Arabs, but I've known some Appys who
excelled in dressage and CT. And you can't beat the elegance of some
Thoroughbreds either. They're good at endurance, too. Just stay
away from the muscle-bound warmblood types. They won't fare well 25
miles down the trail on a hot day.