I've experimented with both, and I found that they both fluoresce certain corals; just a different set for each. The 420 seems to really bring out the greens and the blues, while the 460 brings out the reds.
Also I found that the choice will depend on the color of your main lighting. I tried running 420s with 10k MH (probably lower in reality), and I didn't like the look -- it was not balanced -- just a bunch of yellow and a bunch of purple. The 460s looked good in that case though -- whitenened things up nicely.
With 14k MH (Phoenix, actually higher), the 460s do nothing. The 420s really bring out the colors without creating a disbalanced look.
Bulbs are cheap; I say experiment yourself!
-pavel.