There might only be a psychosomatic difference in flavor, but I'm saying it anyway:
I love purple-fleshed dragonfruit.
I love it so much I have absolutely no problem posting an unflattering picture of me with purple smeared on my nose after chomping into this dragonfruit like an animal instead of walking three train cars down to the dining car and getting a fork.
Where to find dragonfruit in Orange County:
Many places. Dragonfruit is getting more mainstream in the States. They have it at H-Marts, 99 Ranches, Siêu Thị Thuận Pháts, and other big Asian grocery chains around the area. Occasionally I'll even see a few sad specimens in the 'tropical' section at Ralph's.
Where to find purple dragonfruit in Orange County:
Like for good rambutan, you may have to go to the smaller Vietnamese fruit markets (again, anything whose sign says 'trái cây'). I have gotten lucky exactly once, when a close inspection of the leaves yielded me a goldmine of purple goodness.
How to tell the difference:
This is NOT foolproof, but here are some hints:
The leaves on the purple-fleshed ones tend to be a little shorter and more curled. The give in the rind is a little softer: the difference between white-fleshed ones and purple-fleshed ones is like the difference between a ripe Clementine and a ripe satsuma. Finally, the white-fleshed ones tend to be more oval and elongated, while the purple-fleshed ones are squatter.
How to eat:
If you want to be all fancy, you can cut them in half, scoop the flesh out with a spoon, cut it into cubes, and pour the cubes into the skin, which now serves as a bowl. Personally, I just eat it in the messiest, least polite way possible - it gets it into my mouth faster.